Dunmoor Hill

Dunmoor Hill with Cunyon Crags from Knock Hill

Dunmoor Hill with Cunyon Crags from Knock Hill

This excursion provides opportunities to investigate the rock at the granite/lava border. At best, we might find an exposure where the junction is visible but, given the degree to which the Cheviot bedrock is cloaked in peat, we think this is unlikely. It also gives us the chance to compare the rock here with that on nearby Hedgehope Hill. Al-Hafdh identified them as different types, intruded at different times but we are not persuaded that this is so. A third point of interest is an inlier of andesite mapped close to Long Crag on Dunmoor Hill – we are interested in the degree to which it has been altered by the surrounding plutonic rock.

Map showing the route of the Dunmoor Hill excursion with locations and igneous rock types

Key

Key for the Dunmoor Hill excursion map

We park out of the way on the grass at the roadside a short distance up the hill past Greenside Farm, walking back down to the footpath that leads across the rough land to the site of the medieval village at the foot of Dunmoor Hill. As we approach the hill we see a number of rock exposures ranging up the hill topped by the plutonic rock of Cunyon Crag behind. Somewhere between them, the lava/pluton junction runs east-west towards Linhope.

Location 1. Hornfelsed andesite at NT983178 to NT984179

The first exposure of altered andesite, Dunmoor Hill

The first exposure that we investigate on Dunmoor Hill

We begin by investigating some of the exposures that we saw from the track, marked by the Geological Survey as andesite.
The first one we examine, at NT9843178, immediately takes our interest because it is a very hard and variegated rock, parts of which look distinctly granitic in character. We wonder if this is a mixture of ‘Marginal’ plutonic rock and hornfelsed andesitic lava.
Thin sections reveal the main body of the rock is a reddish (haematite-rich), hornfelsed andesite with largely albatised plagioclase phenocrysts and altered pyroxenes together with occasional quartz crystals, set in a fine-grained, opaque-rich ground. The andesite has been intruded by a dark-grey, very fine-grained rock that we are unable to classify, as well as by slightly larger-grained pyroxene-rich veinlets. In these veinlets, the pyroxenes, a mix of ortho- and clino- varieties, appear alongside feldspars together with a little quartz.
The porphyritic host rock clearly pre-dates the intrusive rock and generally speaking, veinlets of the pyroxene-rich cut across the dark fine-grained material which must therefore pre-date it, but in places, the two types of intrusive material appear to have flowed together along cracks in the host.
So, we see no recognisable areas of ‘Marginal’ granitic rock here, but it does seem safe to assume that the presence of the intrusive material, as well as the alteration to the andesite, are all associated with the intrusion of granite.


Andesite hornfels with very fine-grained mafic material and pyroxene-rich veinlets outcropping at Location 1 (NT983178). Prepared hand specimen viewed in reflected light.

Andesite hornfels with very fine-grained mafic material and pyroxene-rich veinlets outcropping at location 1 (NT983178)
Prepared hand specimen viewed in reflected light measuring 30mm across.

Andesite hornfels with very fine-grained mafic material and pyroxene-rich veinlets, Dunmoor Hill. NT983178. Thin section viewed in plane polarised light (Section measures 30 mm across).

Thin section from the same sample viewed in plane polarised light.
This section measures 30 mm across.

Andesite hornfels with very fine-grained mafic material and pyroxene-rich veinlets, Dunmoor Hill. Thin section viewed with crossed polarising filters (Section measures 30 mm across)

The same thin section viewed with crossed polarising filters

Veining across both the altered andesite and the fine-grained intrusion. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Veining across both the altered andesite and the fine-grained intrusion
Diffusion at the edges of the pyroxene-rich veins suggests a hydrothermal in origin.
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Veining across both the altered andesite and the fine-grained intrusion. Section viewed with crossed polarising filters ((FoV 2.3 x 1.5 mm)

The same area of the second thin section viewed with crossed polarising filters

Altered and veined andesite with intrusive rock,, Dunmoor Hill. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

Altered and veined andesite with intrusive rock, Dunmoor Hill
The andesite is in the upper part of the image, the fine-grained intrusive material is below it.
Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

Pyroxene-rich veinlet and fine-grained intrusive material in altered andesite, Dunmoor Hill. Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Pyroxene-rich veinlet and fine-grained intrusive material in altered andesite, Dunmoor Hill
Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)

Pyroxene-rich veinlet and fine-grained intrusive material in altered andesite, Dunmoor Hill. Section viewed with crossed polarising filters (FoV 1.2 x 0.8 mm)

The same area of the thin section viewed with crossed polarising filters
The first-order grey interference colour of the more columnar pyroxene crystals, coupled with there parallel extinction, indicates they are orthopyroxene. These contrast with the more granular, yellow, orange and pink coloured clinopyroxene crystals.
Note the fractured and partially albitised plagioclase phenocryst on each side of the veinlet.

Andesite hornfels with very fine-grained mafic material and pyroxene-rich veinlets outcropping at Location 1 (NT983178). Prepared hand specimen viewed in reflected light.

A second sample from the same location
Prepared hand specimen viewed in reflected light measuring 43mm across.

Andesite hornfels with very fine-grained mafic material and pyroxene-rich veinlets, Dunmoor Hill. Thin section viewed with crossed polarising filters (Section measures 43 mm across)

A thin section from the same sample viewed with crossed polarising filters

The second sample of altered and veined andesite at location 1 viewed in plane polarised light (FoV 4.6 x 3.0 mm)

An area of the second sample of altered and veined andesite viewed in plane polarised light (FoV 4.6 x 3.0 mm)

An area of the second sample of altered and veined andesite viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

The same area of the second thin section viewed with crossed polarising filters
With andesite host rock at the top of the image. it is easy to see the two different intrusive materials below; the dark, very fine-grained one and the more colourful, granular pyroxene-rich one.

Exposure of altered andesite at NT983179, Dunmoor Hill

The second exposure of altered andesite we examine is just to the east of the first, at NT983179

The rock at the second exposure looks more obviously andesitic – it is darker and we see phenocrysts. However, thin sections present us with a surprise. Most of the ‘phenocrysts’ are quartz with few of the large plagioclase phenocrysts that we would have expected to see. The andesite is obviously altered; amphibole minerals have replaced primary mafic minerals, much of the groundmass has been altered to tiny flakes that appear orange in thin section and have parallel extinction, and the presence of so much quartz suggests that it too is secondary.

Andesite hornfels at Dunmoor Hill NT983179. Prepared sample viewed in plain reflected light  (45mm across)

Andesite hornfels at Dunmoor Hill NT983179
Prepared hand specimen viewed in reflected light measuring 45mm across.

Andesite hornfels, Dunmoor Hill. NT983179. Thin section viewed in plane polarised light (Section measures 45 mm across).

Thin section from the same sample viewed in plane polarised light.This section measures 45mm across.

Andesite hornfels, Dunmoor Hill. Thin section viewed with crossed polarising filters (Section measures  45mm across)

The same thin section viewed with crossed polarising filters

Patches of quartz in altered andesite. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Patches of quartz in altered andesite
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Altered and veined andesite, Dunmoor Hill. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

Patches of quartz in hornfels viewed with crossed polarising filters

Amphibole and quartz in altered andesite. Section viewed in plane polarised light (FoV 0.5 x 0.3 mm)

Amphibole and quartz in altered andesite
Section viewed in plane polarised light (FoV 0.5 x 0.3 mm)

Amphibole and quartz in altered andesite. Section viewed with crossed polarising filters (FoV 0.5 x 0.3 mm)

Amphibole in hornfels viewed with crossed polarising filters

Exposure of altered andesite at NT984179, Dunmoor Hill

The third exposure at NT984179, close to, but higher than the second

The rock at the third exposure we sample is hornfelsed andesite. Here, we do see comparatively large plagioclase phenocrysts in a the groundmass that is even more altered. Here are the same, although no quite so extensive, patches of quartz that we saw in the rock at the previous location, and there are also some rounded quartz crystals that lappear primary in origin. The surprise comes in the presence of tourmaline. It is unlike the tourmaline that we have seen elsewhere in the Cheviots in that it is grey-brown rather than the usual dark blue-green and it is less strongly pleochroic.The presence of tourmaline points to hydrothermal alteration on top of the metamorphic effects associated with heat from the nearby pluton.

Andesite hornfels at Dunmoor Hill NT984179. Prepared sample viewed in plain reflected light  (42mm across)

Andesite hornfels at Dunmoor Hill NT984179
Prepared hand specimen viewed in reflected light measuring 42mm across.

Andesite hornfels, Dunmoor Hill. NT984179. Thin section viewed in plane polarised light (Section measures 42 mm across).

Thin section from the same sample viewed in plane polarised light.
This section measures 45mm across.

Andesite hornfels, Dunmoor Hill. Thin section viewed with crossed polarising filters (Section measures  42mm across)

The same thin section viewed with crossed polarising filters

Clinopyroxene altered to amphibole along with a grey-brown mineral, probably tourmaline. Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Clinopyroxene altered to amphibole along with a grey-brown mineral, probably tourmaline.
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

Clinopyroxene (orange)  altered to amphibole along with a grey-brown mineral that is probably tourmaline. Section viewed with crossed polarising filters (FoV 2.3 x 1.5 mm)

The same altered pyroxene viewed with crossed polarising filters

Sieve textured plagioclase, rounded quartz and altered pyroxene crystals in altered andesite. Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Sieve textured plagioclase, rounded quartz and altered pyroxene crystals in altered andesite
Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

Sieve textured plagioclase, rounded quartz and altered pyroxene crystals in altered andesite.  Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

Sieve textured plagioclase, rounded quartz and altered pyroxene crystals in altered andesites. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm) viewed with crossed polarising filter

Large plagioclase phenocryst with some sieve texture in altered groundmass. Flakes appear to exhibit parallel extinction.  Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

A large plagioclase phenocryst showing some sieve texture with smaller phenocrysts set in the altered groundmass
The orange mineral flakes in the groundmass exhibit parallel extinction. Section viewed with crossed polarising filters (FoV 4.6 x 3.0 mm)

Tourmaline in altered groundmass of andesite hornfels. Section viewed in plane polarised light ((FoV 1.3 x 0.8mm)

Tourmaline in the altered groundmass of a second sample from nearby
Section viewed in plane polarised light ((FoV 1.3 x 0.8mm)

Tourmaline in altered groundmass of andesite hornfels. Section viewed with crossed polarising filters ((FoV 1.3 x 0.8mm).

The same crystals viewed with crossed polarising filters

Tourmaline in altered groundmass of andesite hornfels. Section viewed in plane polarised light (FoV 0.6 x 0.4mm)

Tourmaline in altered groundmass of andesite hornfels
The tourmaline crystals that appear in our samples of the hornfels in this area are all this grey-brown in colour. This crystal’s zoning is clearly seen.
Section viewed in plane polarised light (FoV 0.6 x 0.4mm)



Thin-sections of the outcropping rock from this location prove it to be granitic and therefore part of the pluton.  Grain-sizes of the quartz, feldspars, biotite, pyroxenes and opaque iron/titanium ores vary enormously. Often individual crystals and small pieces of medium-grained rock similar to that found higher up the path at Cunyon Crags, are seen suspended in the fine-grained material suggesting the fine-grained component of the rock was injected later than the medium-grained component causing disruption before cooling quickly.

Fine-grained quartz monzonite outcropping at Location 2. prepared hand specimen viewed in reflected light

Fine-grained ‘Marginal’ quartz  monzonite outcropping at Location 2
Prepared hand specimen viewed in reflected light

Fine-grained Marginal quartz  monzonite at Location 2, Dunmoor Hill viewed with crossed polarising filters.

A thin-section from the same sample viewed with crossed polarising filters

Pyroxene and biotite in the groundmass of the same fine-grained Marginal quartz  monzonite. Section viewed in plane polarised light. (FoV 1.3 x 0.8 mm)

Pyroxene and biotite in the groundmass of the same fine-grained ‘Marginal’ quartz  monzonite
The high relief,  clear to pale-green crystals scattered throughout the groundmass are most probably pyroxene. There is also an abundance of fine-grained magnetite. Section viewed in plane polarised light. (FoV 1.3 x 0.8 mm)

Pyroxene and biotite in the groundmass of the same sample viewed with crossed polarising filters. (FoV 1.3 x 0.8 mm)

The same area of the thin section viewed with crossed polarising filters
The mid-first to  second order interference colours suggest  clinopyroxene rather than orthopyroxene. Section viewed in with crossed polarising filters (FoV 1.3 x 0.8 mm)

Altered clinopyroxene in the same fine-grained quartz monzonite viewed in plane polarised light (FoV 2.5 x 1.7 mm)

Altered clinopyroxene  in the same fine-grained ‘Marginal’ quartz  monzonite
There   is no sign of pleochroism in these crystals.
The section is viewed in plane polarised light (FoV 2.5 x 1.7 mm)

Clinopyroxene crystals in  the same fine-grained quartz-monzonite viewed with crossed polarising filters (FoV 2.5 x 1.7 mm)

The same clinopyroxene crystals viewed with crossed polarising filters
The  twinning  that these crystals exhibit along with  roughly 90o  cleavage,  inclined extinction and higher interference colours   are all  typical of clinopyroxene.
(FoV 2.5 x 1.7 mm)

Orthopyroxene at location 2, Dunmoor Hill viewed in plane polarised light. The non-pleochroic crystal, much altered and darkened by opaques, is viewed in plane polarised light.(FoV 2.5 x 1.7 mm)

Orthopyroxene in fine-grained quartz monzonite at location 2
The non-pleochroic crystal, much altered and darkened by opaques, is viewed in plane polarised light. (FoV 2.5 x 1.7 mm)

Orthopyroxene at location 2, Dunmoor Hill viewed with crossed polarising filters. The crystal exhibits fine lamella, low interference colours and straight extinction, all characteristics of orthopyroxene (FoV 2.5 x 1.7 mm).

The same orthopyroxene crystal viewed with crossed polarising filter (FoV 2.5 x 1.7 mm)
The crystal exhibits fine lamella, low interference colours and straight extinction, all characteristics of orthopyroxene (FoV 2.5 x 1.7 mm).

Glomerocryst of plagioclase and augite in the fine-grained matrix viewed at location 2. Section viewed in plane polarised light (FoV 5 x 3.3 mm)

Glomerocryst of plagioclase and augite in the fine-grained matrix viewed at location 2.
Section viewed in plane polarised light (FoV 5 x 3.3 mm).

Glomerocryst of plagioclase and augite in the fine-grained matrix viewed with crossed polarising filters (FoV 5 x 3.3 mm)

The same crystals viewed with crossed polarising filters (FoV 5 x 3.3 mm)

Chlorite vein with actinolite and possibly epidote (with very high relief in upper right of vein) in the same thin section viewed in plane polarised light (FoV1.3 x 0.8 mm)

Chlorite vein with actinolite and possibly epidote (with very high relief in upper right of vein) in the same thin section
Section viewed in plane polarised light (FoV 1.3 x 0.8 mm).

Chlorite vein with actinolite and possibly epidote viewed with crossed polarising filters (FoV1.3 x 0.8 mm)

The same area viewed with crossed polarising filters (FoV1.3 x 0.8 mm)


Intrusion into granitic rock at location 2, Dunmoor Hill. Prepared hand sample viewed in reflected light. The intrusion is the dark material to the right.

Intrusion into granitic rock at location 2, Dunmoor Hill
Prepared hand sample viewed in reflected light. The intrusion is the dark material to the right.

Intrusion into granitic rock viewed with crossed polarising filters.

A thin-section from the same sample viewed with crossed polarising filters

Contact between the more mafic fine-grained intrusion and the red, medium-grained  granitic rock at location 2. Section viewed in plane polarised light. (FoV 5 x 3.3 mm)

Contact between the more mafic fine-grained intrusion and the red, medium-grained, granitic rock at location 2
Section viewed in plane polarised light. (FoV 5 x 3.3 mm)

Contact between the more mafic fine-grained intrusion and the medium-grained, red granitic rock at location 2. Section viewed with crossed polarising filters. (FoV 5 x 3.3 mm)

The same area viewed with crossed polarising filters (FoV 5 x 3.3 mm)


Alteration of mafic material in the red, medium-grained granitic rock giving rise to two forms of biotite along with actinolite and magnetite. One form of biotite, perhaps primary, is a dark brown while the other, most likely secondary, is much lighter in colour. The actinolite is the greenish mineral near the centre of the cluster. Section viewed in plane polarised light (FoV 5 x 3.3 mm)

Alteration of mafic material in the medium-grained granitic rock giving rise to two forms of biotite, actinolite and magnetite
One form of biotite, perhaps primary, is  a dark  brown while the other, most likely secondary, is much lighter in colour. Section viewed in plane polarised light (FoV 5 x 3.3 mm)

Alteration of mafic material in the red, medium-grained granitic rock giving rise to two forms of biotite along with actinolite and magnetite at location 2.  Section viewed with crossed polarising filters(FoV 5 x 3.3 mm)

The same group of crystals viewed with a slight rotation of the stage and with crossed polarising filters (FoV 5 x 3.3 mm)

Location 3. Cunyon Crags NT977180

Cunyon Crags NT 97704 18099 presents some of the best exposures of the Cheviot’s dark grey, medium-grained ‘Marginal’ quartz monzonite rock.

Marginal quartz monzonite at location 3, Cunyon Crags. Prepared hand specimen viewed in reflected light.

‘Marginal’ quartz monzonite at Cunyon Crags
Prepared hand sample viewed in reflected light

Marginal quartz monzonite at location 3, Cunyon Crags viewed with crossed polarising filters

A thin-section from the same sample viewed with crossed polarising filters

Location 4. Marginal quartz monzonite at NT970177


Next, there’s a long boggy stretch that crosses, according to Robson’s map, the boundary between the outer ‘Marginal’ quartz monzonite and the ‘Central Belt’ medium-grained,  porphyritic granites that lie closer to the centre of the area occupied by the pluton. Sadly, we are unable to find any exposures where we can see this junction.
We are heading for an outcrop at NT970178 where Al-Hafdh records his ‘Hedgehope’ granodiorite chilled against the ‘Marginal’ quartz monzonite and therefore, evidence for sequencing the intrusions. We spend time carefully observing the outcrops in the vicinity but we do not find find a chilled margin.
However, our search does reveal a wonderful variety in the ‘Marginal’ rocks in quite a small area at this locality, evidencing a seemingly chaotic interaction of differently constituted magmas. Over a few square metres, we find; rock that is wholey medium-grained ‘Marginal’ quartz monzonite, ‘Marginal’ rock that has been invaded by a fine-grained relatively mafic-rich material, some strongly granophyric rock, and also rocks intruded by fine-grained mafic-poor material.

Fine-grained Central Belt granitic rock at location 4. Prepared hand sample viewed in reflected light

Fine-grained ‘Central Belt’ granitic rock at location 4
Prepared hand sample viewed in reflected light

Fine-grained Central Belt granitic rock at location 4 on Dunmoor Hill viewed with crossed polarising filters.

Thin section from the same  sample viewed in transmitted light with crossed polarising filters

Medium-grained Central Belt rock at location 4 on Dunmoor Hill, Prepared hand sample viewed in reflected light

Medium-grained ‘Central Belt’ rock  at location 4 on Dunmoor Hill
Prepared hand sample viewed in reflected light

Medium-grained Central Belt granitic rock at Location 4 viewed with crossed polarising filters

A thin-section from the same sample viewed with crossed polarising filters

Fine-grained felsic vein intruded into sub-equigranular Central Belt rock. Prepared hand sample viewed in reflected light.

Fine-grained felsic vein intruded into sub-equigranular ‘Central Belt’ rock  at location 4 on Dunmoor Hill
Prepared hand sample viewed in reflected light.

Fine-grained felsic vein intruded into sub-equigranular Central Belt rock at location 4 on Dunmoor Hill. Thin section viewed with crossed polarising filters.

Thin section from the same sample viewed with crossed polarising filters

A sub-equigranular, mafic-rich granitic rock disrupted by a fine-grained intrusion that is also mafic-rich. Prepared hand sample from location 4 on Dunmoor Hill.

A sub-equigranular, mafic-rich granitic rock disrupted by a fine-grained intrusion that is also mafic-rich
Prepared hand sample from location 4 on Dunmoor Hill.

A sub-equigranular, mafic-rich granitic rock disrupted by a fine-grained intrusion that is also mafic-rich. Thin section viewed with crossed polarising filters.

Thin section from the same sample viewed with crossed polarising filters.


Location 5. Dunmoor hill summit NT967183


Boggy track leading to summit tors of Dunmoor Hill

The rough track leading towards the summit of Dunmoor Hill
The summit tors can be seen on the skyline.

From location 6 we take the footpath up to the summit of Dunmoor Hill where there are some small exposures. The rock here is quite different from Cunyon Crags, being a fine-grained to medium-grained ‘Central Belt’ type.

View from Long Crag of distant tors of coarser porphyritic

‘Central Belt’  tor on Long Crag.

Location 6. Long Crag at NT967178

Granite jointing at Long Crag

Outcrop at Long Crag
The rock shows typical granite tor structure and jointing.


Long Crag lies on a north-south axis from NT 967178 providing more interesting exposures of rock.
The rock is porphyritic, commonly with larger plagioclase and potassium feldspar phenocrysts and smaller biotite ones. Occasionally we find large pyroxene glomerocrysts. The matrix shows a variety of grain-sizes, from very fine to medium, and commonly contains the ragged remains of pyroxene crystals, along with opaque iron/titanium oxides.

Medium-grained Central Belt rock at location 6 at the northern end of Long Crag. Prepared hand sample viewed in reflected light.

Medium-grained  ‘Central Belt’ rock  at location 6 at the north end of Long Crag
Prepared hand sample viewed in reflected light

Medium-grained Central Belt rock at location 6 at the northern end of Long Crag viewed with crossed polarising filters.

A thin section from the same sample viewed with crossed polarising filters

Fine-grained Central Belt rock at the south end of Long Crag, location 6. Prepared hand specimen viewed in reflected light

Fine-grained  ‘Central Belt’ rock  at the south end of Long Crag, location 6
Prepared hand specimen viewed in reflected light

Fine-grained Central Belt rock at the south end of Long Crag, location 6 viewed with crossed polars.

A thin-section from the same sample viewed with crossed polarising filters

Location 7. Long Crag at NT968174

The rock here has a very fine-grained red matrix that surrounds sparser feldspar, biotite and pyroxene phenocrysts. The matrix suggests the rapid cooling that would occur if this magma was chilled by its close proximity to the already consolidated ‘Marginal’ quartz monzonite.

Rock with a fine-grained matrix with sparser phenocrysts at location 7, Long Crag. Prepared hand specimen viewed in reflected light.

Rock with a fine-grained matrix with sparser phenocrysts at location 7, Long Crag.
Prepared hand specimen viewed in reflected light.

Rock with a fine-grained matrix with sparser phenocrysts at location 7, Long Crag, viewed with crossed polarising filters.

Thin-section from the same sample viewed with crossed polarising filters

Location 8 Cat Crag NT 967172

South slope of Dunmoor Hill. The junction between diorite (right) and pink (left)

Cat Crag
At NT 96801 17553, there is a junction between ‘the Marginal’ quartz monzonite and a pink medium-grained porphyritic rock. The pink rock lies to the left in this photograph, and the quartz monzonite to the right.

Return to car

From Cat Crag we head down over rough ground to rejoin the path back to Linhope and from there, the road to Hartside Farm and the car.

References

Al-Hafdh N.M. 1985. The Alteration Petrology of the Cheviot Granite. Thesis submitted for PhD. at Newcastle University.

British Geological Survey  Online geology map http://mapapps.bgs.ac.uk/geologyofbritain/home.html

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