18 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 22, 



collected as to the dates of the igneous eruptions of Strath. 1st, 

 they are all posterior to the Middle Lias, being intrusive and not con- 

 temporaneous ; 2ndly, the first period of igneous action gave rise to a 

 great profusion of trap-dykes, which intersected every part of the 

 district ; they did not, however, produce any marked alteration of 

 the general stratification and texture of the rocks, but rather con- 

 formed themselves to the natural joints of the beds, and hence ac- 

 quired a prevailing strike to north-west ; 3rdly, subsequently vast tracts 

 of syenite tilted up the liassic strata, in a manner analogous to that 

 of the Arran granites, and produced in them an extensive and com- 

 plete metamorphism ; 4thly, the last period of igneous action was 

 characterized by the outburst of great hills of fine-grained syenite, 

 that intruded itself among the beds without either tilting them or 

 producing any considerable amount of alteration upon them ; at the 

 same period sheets of greenstone were occasionally thrust among the 

 syenites and limestones, or broke through them as massive dykes*. 



Metamorphism. — The general arrangement and effects of the 

 igneous rocks having been pointed out, I shall proceed to describe 

 the metamorphism so often alluded to. The metamorphic district, 

 as may be conjectured, lies among the disruptive syenites. Its north- 

 western limit is formed by the great syenitic chain of Beinn Dhearg 

 and Beinn na Cailleaich ; the hillward slope of Beinn Shuardail and 

 the hollow of Glen Kilbride bound it on the south-east ; the shore 

 of Loch Slapin, from Beinn na'a Cro to the mouth of Glen Kilbride, 

 forms its south-western termination ; while to the north-east it ap- 

 proaches Beinn Bhuidhe and Broadford. Within this area the 

 bedded limestones of Broadford Bay are altered into a crystalline 

 mass ; stratification is usually obliterated, along with all trace of 

 fossils, and what was before a dull blue limestone assumes all the 

 sparkle and varied tints of a primary marble, mottled sometimes like 

 that of Balahulish, or pure like that of Carrara. 



The action of atmospheric agents upon this rock has caused it to 

 weather in a very singular manner. The eminences which it forms, 

 in not a few instances, look as if a shower of oblong grey blocks had 

 been shot into their soft peaty surfaces. In some places the marble 

 assumes a finely cavernous exterior, pitted all over, like a sandy 

 beach after rain. At other times the blocks are smooth as tomb- 

 stones, and stand out in bold relief from the heath and furze that 

 surround their base. No moss or lichen can cling to them, nor does 

 the vegetation of the soil cluster up their sides, as it never fails to do 

 round the mouldering dykes, so that the marble- hillocks arrest the 

 eye at once, and can be distinguished at a considerable distance. 

 This manner of weathering is not, however, peculiar to the marble. 

 It may be seen, though in a much less marked manner, along the 

 outcrop of unaltered limestone south of Sculamus. But the most 



* The geological era of these events has still to be fixed. Professor Edward 

 Forbes was of opinion that the trap-hills of the north-east of the island were of 

 Middle Oolite age, but admitted that the district would require a more extended 

 investigation before the point could be made out with certainty. See Quart. 

 Journ. Geol. Soc. vol. vii. p. 109. 



