ANNIVERSAUY ADDRESS OF THE PRESIDENT. l6l 



tion by Jukes, is an iuvaluable guide to the geologist who would 

 explore this somewhat inaccessible region *. The most important 

 correction that will require to be made in the work arises from a 

 mistake as to the true nature of certain rocks which were described 

 as pisolitic tuffs, but which are nodular felsites. 



By far tho most striking geological feature of this singularly 

 interesting and impressive coast-line is to be found in the interstra- 

 tification of lavas with bands of tuff among abundantly fossiliferous 

 strata which, from their organic contents, are unmistakably of the 

 age of the Wenlock group. These lavas occur in a number of 

 sheets, separated from each other by tuffs and other fragmental 

 deposits. They thus point to a series of eruptions over a sea- 

 bottom that teemed with Upper-Silurian life. They consist for the 

 most part of remarkably fine typical nodular felsites. The nodules 

 vary in dimensions from less than a pea to the size of a hen's egg. 

 They are sometimes hollow and lined with quartz- crystals. They 

 vary greatly in number, some parts being almost free from them and 

 others entirely made up of them. The matrix, where a fresh 

 fracture can be obtained, is horny in texture, and often exhibits an 

 exceedingly beautiful and fine flow-structure. On weathered faces 

 there may be seen thick parallel strips and lenticles of flow-struc- 

 ture like those of the Snowdon lavas. The upper portions of some 

 of the sheets enclose fragments of foreign rocks. The microscopic 

 examination of a few slices cut from these lavas shows them to be 

 true felsites (rhyolites) composed of a microcrystalline aggregate of 

 quartz and felspar, with layers and patches of cryptocrystalline 

 matter, and only occasional porphyritic crystals of orthoclase and 

 plagioclase. 



The pyroclastic rocks associated with these lavas vary from 

 exceedingly fine tuff" to coarse agglomerate. Some of the finer tuffs 

 contain pumiceous fragments and pieces of grey and red shale ; 

 they pass into fine ashy sandstones and shales, crowded with 

 fossils, and into gravelly breccias made up of fragments of diffe- 

 rent volcanic rocks. 



But the most extraordinary of these intercalated fragmental 

 strata is a breccia or agglomerate, about 15 feet thick, which lies in 

 a thick group of fossiliferous dull-yellow, ashy, and ochreous sand- 

 stones. The stones of this bed consist chiefly of blocks of different 

 felsites, varying up to three feet in length. Some of them show 

 most perfect flow-structure ; others are spongy and cellular, like 



* Sheets 160 and 171 of the one-inch map, and Memoir on Sheets 160, 161, 

 171, and 172. 



