408 EECENT WOEK OE THE GEOLOGICAL SUEVEY 



always described as being vertically piped by ScoUthus like tbe 

 " pipe-rock," and (2) the Calciferous Group; in other words, the 

 highest beds of the Cambrian and the lowest members of the Silurian 

 formations. There can be little doubt that some old shore-line or 

 shallow sea must have stretched across the North Atlantic or Arctic 

 ocean, along which the forms migrated from one province to the 

 other, and that some barrier must have cut off this area from that of 

 Wales and Central Europe. 



ly. Igneous Eocks ix Cambeiaij and Siltjeian Poemations. 

 1. Evidence of their Intrusive Character. 



The Lower Palaeozoic strata of Assynt furnish evidence of an 

 outburst of volcanic activity after the deposition of the Limestone 

 series. The crystalline rocks which contain the records of this 

 episode in the geological history of the North-west Highlands are all 

 intrusive and occur in the form of sheets extending for miles along 

 the bedding -planes. 



That these igneous rocks are intrusive and not contemporaneous 

 will readily be admitted for the following reasons : — first, when the 

 sheets are followed along the line of outcrop they pass transgressively 

 from lower to higher members of the same group ; second, where 

 they reach a considerable thickness, both the oveiiying and under- 

 Ij'ing strata are altered by contact-metamorphism ; and third, they 

 frequently contain patches of the sedimentary beds which they have 

 traversed, as, for example, fragments of altered quartzite in the 

 diorites associated with the limestone. 



2. Horizons. 



The phenomena presented by these intrusive sheets are admi- 

 rably displayed on the slopes of Ben Garbh and Canisp, south of 

 Loch Assynt, where they have been injected along the bedding-planes 

 of the Cambrian sandstones and Silurian quartzites, and again in 

 the great limestone cliff at Stronechrubie (Inchnadamff) on the 

 horizon of the Ghrudaidh Group. Por long distances the foregoing 

 masses keep to the same horizon, even where the strata are dipping 

 at low angles, but eventually they leave it and pierce the beds above 

 or below. On the western face of Canisp, a large mass of porphy- 

 ritic felsite rises from the old platform of Archaean rocks, passing 

 upwards into the Cambrian sandstones and ultimately spreading 

 along the bedding-planes of the strata. Several important sheets 

 are also found on higher horizons both on Canisp and Ben Garbh. 



The foregoing vertical Section II. (fig. 7) shows the prevalent hori- 

 zons of these intrusive masses in the Silurian rocks of Assynt, from 

 which it win be seen that they occur in both zones of the quartzites, 

 the " Fucoid-beds," and in the two lowest limestone groups. It ought 

 to be remembered, however, that though they generally occupy the 

 particular horizons indicated in the table, they not unfrequently 

 appear either in higher or lower subdivisions of the same group. 



