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208 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. '[Feb. 6, 



Atlantic. The slate is cleaved obliquely to tbe bedding, the planes 

 having, where we saw them, a marked S.E. slope. This rock weathers 

 in a remarkable way along the precipitous headland of Sanaig. Seen 

 through the mist, the hills seemed split open by deep narrow fissures, 

 corresponding probably to joint-planes, while their steep faces had 

 that peculiar jagged aspect which so often results from the weathering 

 of cleavage and stratification combined. 



If we are correct in assigning to these slates a position inferior to 

 the grits and schists which in Islay represent the lower quartz-rock 

 of Sutherland, it follows that they help to fill up a gap in the Silu- 

 rian series which is wanting in the North. The Sutherland rocks 

 unquestionably belong to the lower part of the Llandeilo series, and 

 the Sanaig slates would accordingly represent the position of the 

 Lingula-flags. 



The slates are followed towards the east by what we at present 

 regard as a superior series of schists and intercalated bands of 

 quartzose grit. From this point there is no doubt about the order 

 of succession. The strata clip towards the south-east, — the schists 

 occupying, as a rale, the valleys and low grounds, while the bands 

 of hard grit range along the island in more or less prominent ridges, 

 or even rising up into no inconsiderable hills. The schists do not 

 differ from those which have been already described ; they . are 

 micaceous, chloritic, argillaceous, or quartzose in turn, and often 

 much contorted in their lamination. The grit or quartz-rock is a 

 hard grey siliceous rock, to which the old term " grauwacke" might 

 well be applied. These strata occupy the whole of the western 

 division of Islay, known as " the Rhinns," — the schistose series being 

 sometimes quarried for slates, as at Kilchieran, and the grits as 

 rough building-stones. Granitoid rocks and true syenites are of 

 common occurrence, and may be advantageously examined along the 

 south-east side of the promontory at Port Charlotte. There, on the 

 shores of Loch-in-Daal, the quartzose grauwacke and grey schist, 

 dipping S.E. at 40°, are traversed by a red syenite, and are them- 

 selves highly metamorphosed, passing into quartz-rock and lydian- 

 stone, in which a crystal of felspar may be occasionally detected. 

 The decomposition of the syenite here has produced that chaotic 

 assemblage of loose blocks (felsen-meer) so common in granitic 

 countries. Dykes of greenstone with a general north-west range, 

 but often irregular and tortuous, also occur among the strata of this 

 coast-line as well as throughout the entire island. 



As we trace these schists and grits towards the N.E. promontory 

 of Islay, the former are seen to become considerably thinner, and the 

 series then assumes much more of a quartzose aspect. This was 

 particularly noticed on the high grounds to the north of Islay 

 House. 



The section (fig. 20) shows that above the grits there is a series of 

 schists with three intercalated limestones. This upper series we 

 refer to the horizon of the Assynt Limestone. It lies upon a quartzo- 

 schistose band, representing, as we have said, the lower quartz-rock 

 of Sutherland. It is surmounted by a great mass of white quartz- 



