THE GEOLOGY OF GARABAL HILL.

IX.-THE GEOLOGY OF GARABAL HILL.

By ALEXANDER SCOTT, M.A., B.Sc.

The complex of plutonic rocks, which forms the belt of highground stretching from Inverarnan southwards to the head of GlenSloy and westwards to Glen Fyne, was first described by Tealland Dakyns in 1892. The country rock consists mainly of mica­schist, with occasional bands of albite-schist and intrusions ofamphibolite. The mica-schist shows some considerable variety,and varies from a coarse granulitic gneissose rock to a fine-grainedmuscovite-schist, consisting entirely of mica, felspar, and quartz.Large veins and strings of quartz are very common. The area inStrath Dubh Uisge, mapped by the Survey as "Green-beds," isoccupied by a rock which is a typical amphibolite and is made upof hornblende and felspar, with subordinate epidote, zoisite andmica. As suggested by Professor Gregory (1910), it is muchmore like a metamorphosed basic igneous rock than the " Green­beds." The latter are hornblendic in the Glen Fyne district,but the Strath Dubh Uisge rock differs from these in the absenceof bedding planes and other evidence of clastic structure. Thefield relations, too, suggest an intrusive igneous origin. Similarrocks occur on the top of Stob an Fhithich, and also on Meallnan Caora, three miles to the north-west.

Garabal Hill is occupied mainly by the boss of plutonic rocks,the composition of which varies from tonalite and mica-diorite toserpentine. This boss is broken on the west side by a N.N.E.fault, which has shifted the diorites some distance to the south.The high ground of Meall Breac, lying between this fault andGlen Fyne, is occupied by porphyritic granite. The plutonic rocksare covered in many places by a thin layer of schist, which isprobably a residuum of the original schist-roof. The schists nearthe diorite often show the effects of contact metamorphism, andhave suffered granulitisation, with occasional development ofandalusite. Along the N.N.E. fault the rocks have been verymuch shattered with the formation of a very fine-grained felspathicgranulite. The ultrabasic rocks occur in a number of isolatedoutcrops along a line running N.N.E. from Loch Garabal, thelargest exposure covering about one-eighth of a square mile, andlying immediately north ofthe loch (loc. i). Another exposure ofabout 200 square yards appears a little to the east of LochanBeinn Damhain (loc. ii), while several smaller ones are foundfarther to the north (loc. iii). The predominant rock type is apyroxenite consisting essentially of large bronzy diallage crystals.Olivine is often present, and in places becomes predominant. Acontinuous sequence from a pure pyroxenite through wehrlite(olivine-augite rock) to dunite can be traced. Hornblende occursPROC. GEOL. Assoc., VOL. XXVI, PART 3, 1915.] 13

ALEXANDER SCOTT.

in varying amount and usually increases towards the margins. Acolourless enstatite and biotite are sometimes found. At loco iithere occurs a rock which has been termed davainite, and which

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consists almost entirely of glossy black crystals of hornblende.In thin section, the crystals are brown and remarkably uniform insize. The cores of the crystals are often composed of diallage,

THE GEOLOGY OF GARABAL HILL.

and the rock seemed to have been formed by the alteration of apyroxenite. This explanation is favoured by the fact that italways intervenes between the diorite and the ultrabasic rock.The other minerals of the latter are all found in davainite, withthe exception of olivine, which seems to be represented bynumerous grains of strongly pleochroic pink hypersthene. Locally,in the ultrabasic rocks of loco i, veins of very coarse wehrliteoccur, and these seem to be of the nature of peridotite­pegmatites.

The diorite forms a strip, up to half a mile in width, along theeastern margin of the boss, and is bounded on the west by thetonalite. The latter also occurs to the west of the fault, where itappears to pass gradually into porphyritic granite. The porphy­ritic granite is characterised by large crystals of orthoclase in aground-mass of quartz, felspar, and biotite. The tonalite consistsof strongly pleochroic biotite and zoned oligoclase in a matrix ofquartz, orthoclase, and plagioclase.

The diorites show considerable variation, and grade fromquartz-mica diorites to pyroxene-diorites and hornblendites.North of Garabal Hill the rock is sufficiently basic to be called ahornblende-gabbro. In the neighbourhood of loco iii numerouscrush-veins occur, with the formation of thin bands of epidosite.At loco iv and loco v remarkably coarse rocks occur. A contin­uous passage can be traced from an even-grained pyroxene-mica­diorite to a hornblendite with 85 per cent. of hornblende,the crystals of which average about one inch in length. Thishornblende, which is brown, has formed from the alterationof a green hornblende, and differs considerably from the davainitemineral.

With regard to the field relation of the rocks there is clearevidence that the ultrabasic rocks are the earliest, the next in­trusion being the diorite, which has been followed by the tonaliteand granite. The final products of consolidation are thenumerous aplite veins and a few dykes of decomposed felsparporphyry. The lithological evidence would suggest that theigneous rocks belong to the series of plutonic intrusions of lowerOld Red Sandstone age, which are common throughout theHighlands.

The whole district has suffered intense glaciation. Thetraces of the main glaciation can be seen in numerous glaciatedsurfaces, with strise indicating a N.W.-S.E. trend of the ice. Suchstrise occur on the east side of Garabal Hill, and north of LochGarabal. A similar ice-trend is indicated by many of meboulders in the lower part of Strath Dubh Uisge. There is alsoevidence that smaller local glaciers came down the valleys,as fine moraines occur in Strath Dubh Uisge and in theFalloch Valley opposite the outlets of the various tributaryvalleys. Most of the boulders in the moraines consist of local

186 THE FALLS OF CLYDE.

rocks. Both the Falloch Valley and the Strath Dubh Uisge aretypical U-shaped valleys, and the former has been considerablyover-deepened as the Strath Dubh Uisge is a hanging valley some700 ft. above the level of the Falloch. Similar hanging valleysoccur on the other side of the Falloch. It is very probable thata shallow moraine-dammed lake occupied the floor of Strath DubhUisge before the river had cut its way through the moraines.

REFERENCES.

Geol. Surv. Maps. Sheets 38,45,46.1892. DAKYNS and TEALL.1913. WYLLIE and SCOTT.1886 and 1913. CADELL.1910 and 1915. GREGORY (and FIORDS, 1913).

X.-THE FALLS OF CLYDE.

By JAMES STARK.

The terrain around the town of Lanark affords a classicexample of certain changes in the physiography of a districtoccasioned by ice-action during the Glacial Period. Previous tothat epoch the Clyde Valley above Lanark was wide and open, asit still is, a couple of miles below that town, where the 600-ft.contour lines are about four miles apart, and the bottom of thevalley is some 400 ft. below that datum.

The glaciation of the district has been mainly from theSouthern Uplands, as the vast majority of the stones in the drifthave come from that direction; only a small proportion havecome from the Highlands in the north-west. As this part of theold Clyde Valley lay across the glaciation of the district, it, as alsothe valleys of some of its tributaries, has been entirely filled upwith drift, the upper portion of which consists of an immenseaccumulation of gravel and sand, covering an area of manysquare miles in extent.

At the close of the .Glacial Period, the Clyde, in this part ofits course, taking the lowest levels among the surface inequalitiesof the drift, appears to have begun the -excavation of a newchannel at the point where it encountered the rocky boundary ofits ancient valley below Braxfield, there forming a waterfall,which, cutting its way backwards, has excavated the defile belowNew Lanark. At the same time another fall must have com­menced a short distance below the present position of CorraLinn, where the river, in its new channel, traversed the margin ofwhat may have been the pre-glacial valley of the Douglas Water,the present confluence of which with the Clyde is about two