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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Feb. 6, 



The junction of the quartz-rock with these schists can be seen at 

 many places. Thus the sea has cut some instructive sections near 

 Macarthur's Head, and still more clearly at the Mull of Oe. The 

 streams too which descend from the hills in an easterly direction have 

 laid open many exposures of the rocks. The jimction-line of the 

 two series of strata is not a sharp one. The quartz-rock passes, by 

 intercalations of schist, into the schistose group, and the latter, by 

 interstratifications of quartz-rock, into the quartzose group. The 

 structure of this part of the island vividly recalls that of the east 

 coast of the peninsula of Sleat in Skye. 



The schists which thus rest upon, and at their base are interlaced 

 with, the quartz-rock vaiy in character, being sometimes micaceous, 

 argillaceous, quartzose, chloritic, or talcose. These changes of litho- 

 logical distinction are so rapid that it is impossible to give any one 

 mineralogical name to the group, or to map out its members by 

 mineral characters. Some of the argillaceous beds are quarried for 

 slate in Port Ellen Bay, where they have a rude cleavage. Some of 

 the harder beds of grit which occur among the schists have a green- 

 ish colour and a slightly chloritic composition, and (as at Port Ellen) 

 have been worked as building- stones. Sometimes the schists are 

 varied by the intercalation of hard micaceous and siliceous flagstones, 

 which are well exposed along the coast at Ardimersay, the residence 

 of our very kind friend Mr. liamsay of Kildalton. One thin course 

 of limestone occurs at Kintore. 



But the most marked feature of this schistose series is the great 

 number and extent of its hornblendic greenstones. These are 

 grouped in long parallel ridges corresponding to the strike of the 

 strata. They are, however, beyond all doubt intrusive sheets, since 

 they alter the rocks along the line of contact, and do not always 

 conform to the bedding, but here and there cut across it, twisting 

 and hardening the surrounding schists*. The same greenstones 

 occur also in the quartz-rock, but sparingly. Their great develop- 

 ment is among the schists, where, along the coast from Port Ellen to 

 near Macarthur's Head, they form the more prominent reefs and 

 islets, to which they give a narrow outline and a north-east and 

 south-west direction. 



The whole of the Island of Islay is more or less traversed by a 

 series of basalt-dykes, which range in a general way from north- 

 west to south-east — a direction which characterizes all the later 

 dykes of the western coast of Scotland. These igneous rocks cut 

 across every other rock in the island. Their posteriority to the 

 greenstones is beautifully shown on the coast south of Macarthur's 

 Head, where the long crags of greenstone which run out to sea are 

 crossed by veins of basalt that pass from reef to reef with striking 



observations on Isla would probably determine the true relative era of the 

 quartz-rock, and elucidate still further the disposition of Schihallien, of Jura, 

 and of the north of Scotland." 



* Since our observations were made, Mr. Jameson of Ellon has examined the 

 geology of the opposite mainland of Cantyre, and found that the schists there 

 are traversed by a similar series of intrusive beds of greenstone, which, as in 

 Islay and Jura, are crossed at right angles by a newer group of basalt-dykes. 



