406 



E. KT7TLEY ON THE MICROSCOPIC STRUCTURE OF 



fine strings or films. Eig. 1 conveys a very fair idea of the general 

 appearance of the section as seen by ordinary transmitted light, and 

 magnified 55 diameters. Between crossed Nicols the shreds and 



Fig, 1. — Fissile Schist from near Pont-y-Gromlech. 

 {Magnified 55 diam.) 



films are shown to be crystalline granular aggregates, which are in 

 great part quartzose. Under a power giving about 250 diameters, 

 the matter lying between the shreds appears to consist chiefly of 

 very minute doubly-refracting granules ; and I think that the whole 

 rock simply presents different textural conditions of felstone, im- 

 pregnated with some mineral of a chloritic or serpentinous character, 

 possibly antigorite, since a porphyritic and schistose rock occurring 

 just above Llyn Teyrn contains better-developed films of a similar 

 character which closely resemble antigorite. The principal point of 

 interest, however, is the meaning of the confused assemblage of 

 many-shaped shreds which impart a distinctive character to thin 

 sections of this rock. I think that a microscopic examination of 

 the rhyolite from Gardiner's River, in Montana, U. $., described and 

 figured in the paper on the rocks of that country which has already 

 been laid before the Society, will suffice to show that it is approxi- 

 mately the same as the rock now under discussion. 



I do not believe that the structural peculiarities by means of 

 which the osteologist correlates the bones of one animal with those 

 of another, are more trustworthy than the structural peculiarity 

 visible in these two rocks. A thin section of a deep-red obsidian 

 from Tolcsva in Hungary, given me by Professor Judd, is placed on 

 the table for comparison with these rocks." "When magnified (fig. 2), the 

 pale glass which constitutes the ground-mass of the section is seen 

 to be filled with minute strings of a reddish-brown glass, twisted in 

 the most irregular manner. It will also be seen that the planes of 



