Vol. 50.I ANNIVERSARY ADDRESS OF THE PRESIDENT. IO9 



being, as he observes, found likewise over an area extending from 

 Yorkshire on the east to Donegal on the west. He does not 

 hesitate to apply the term ' volcanic ' to a series of infilled fissures, 

 whose subaerial products have been so insignificant that conspicu- 

 ous traces of them are only known to exist at a few points, such 

 as Ben Hiant in Ardnamurehan and the Scuir of Eigg. The 

 chief types of these late ejections are represented by a series of 

 rocks, such as the Cleveland dyke and similar eruptive masses, 

 ranging from augite-andesites to dolerites ; in these there is not 

 much glass, as a rule, but occasionally this may be present in such 

 quantity as to produce a pitchstone-porphyry. The other type of 

 rock is represented by the various ' pitchstones ' of the Western Isles 

 of Scotland. These rocks are of much more acid composition than 

 the before-mentioned augite-andesites, and their silica-percentage 

 ranges from 65 to 75, the vitreous groundmass being usually in 

 excess ; by devitrification they pass into various forms of felsite 

 and quartz-felsite. 



In the study of these dykes Prof. Judd considers that there is 

 abundant evidence of the process of differentiation of lavas. Those 

 of Arran are especially instructive, as affording proofs of the 

 re-opening of the fissure after its first injection and the introduction 

 of materials of a totally different composition. Hence the differ- 

 entiation in these cases must have taken place previously. One of 

 the most interesting examples of the union of the two types of late 

 Tertiary lavas, viz. the basic augite-andesite and the acid pitch- 

 stone, in a single dyke, occurs in the great mass of Tertiary granite 

 occupying the northern half of the Island of Arran. This is known 

 as the Cir Mhor dyke, which has a felsite-and-pitchstone centre, 

 whilst the sides are composed of a porphyritic augite-andesite, the 

 acid and basic rocks being always completely distinct, though 

 varying in relative width. They are also strongly contrasted with 

 each other, alike in the characters of all their porphyritic crystals 

 and of their vitreous bases. The acid rocks, however, contain a 

 few crystals evidently derived from the basic rock, which in this 

 case must have been the first to come up. The microscopical 

 characters of the rocks composing the Cir Mhor dyke are described 

 with considerable detail, and mention is also made of hyalite as a 

 constituent of the acid portion of the dyke. 



On the shore and in the cliffs at Tormore, on the west coast of 

 Arran, there occurs a remarkable plexus of dykes, many of which, 

 according to Prof. Judd, supply striking illustrations of that class 



