414 J. CLIFTON WARD ON THE MICROSCOPIC STRUCTURE OF 



of rock " *, we might rather suspect that the thickness of the over- 

 lying Upper Silurians must have been greater than the estimate above 

 given, supposing Sorby's general inference to hold good. I should 

 like to be able to take up this branch of the subject at some future 

 time, unless some more competent hands would undertake itf . 



e. Parallel Metamorphism of Welsh Lavas and Ashes. — If I am right 

 in regarding the formation of the felstone-like beds in the Lake district 

 as a result of metamorphism of felspathic ashes, it seems only rea- 

 sonable to conclude that similar effects in Wales have been produced 

 by a like process. And when one considers the clear geological 

 evidence which Prof. Ramsay has brought forward to show that, 

 1, the Lower Silurian rocks were contorted and cleaved, and there- 

 fore altered, before the deposition of the Upper Silurian, and, 2, that 

 when covered by a great thickness of Upper Silurian beds, they ex- 

 perienced another great disturbance, we may well conclude that the 

 old felspathic ashes and traps must have undergone great alteration. 



Prof. Ramsay has most clearly demonstrated that the felstono 

 sheets of the Arans and of Snowdon are contemporaneous, i. e. that 

 they represent old lava-flows. The nearest modern representatives 

 of these old flows are doubtless the trachytes ; thus Durocher gives 

 the following as the means of analyses by different experimenters. 



Trachyte. Felstone. 



Silica 72-8 75-4 



Alumina 15-3 15-0 



Potash 6-4 3-1 



Soda : 1-4 1-3 



Lime 0-7 0-8 



Magnesia 0-9 1*1 



Oxides of Iron and Manganese . . 1*7 2*3 



Loss by ignition 0-8 1-0 



100-0 100-0 



Are we not justified, then, in supposing that these old felstones 

 had once the characters of modern trachytes $, and that they have 

 undergone great changes when deeply buried and subsequently, 

 giving them their present aspect? Every year the doctrine of 

 uniformity in the operations of nature, as advocated by Lyell, 

 becomes stronger. Basaltic rocks are now no longer regarded 

 (thanks in great part to the labours of Allport) as never older than 



* " Structure of Crystals," Quart. Journ. Geol. Soc. vol. xiv. p. 494. 



t Since this paper was read, I have made a preliminary examination on this 

 subject ; and the results have been submitted to the Society. Prof. Geikie (Trans. 

 Edin. Geol. Soc. vol. ii. pt. 3, 1874) believes that the metamorphism of the Lower 

 Silurian rocks of the Highlands could not have taken place beneath more than 

 5000 feet of Upper Silurian strata. He thus differs entirely from Mr. Sorby, 

 who believed the fluid-cavities to indicate the consolidation of the Highland 

 granites under a pressure equivalent to that of 76,000 feet of rock. 

 r % Probably rather of the group of quartz-trachytes than of that of the less 

 si'licated. 



