DEVITRIFIED ROCKS FROM BEDDGELERT AND SNOWDON. 



407 



section cut off parts of these convoluted strings, leaving irregular- 

 shaped shreds similar to those in the rocks from Gardiner's River 

 and Pont-y-Gromlech, only very much smaller. The shreds in the 

 latter rock appear, therefore, to be parts of convoluted strings or 

 bands. 



We are thus comparing the Pont-y-Gromlech rock with a distinctly 

 vitreous rock (obsidian of Tolcsva), on the one hand, and with a rock 

 which does not present a glassy aspect (rhyolite of Gardiner's River), 

 on the other. A question of great interest now arises. "Was the 

 rhyolite of Gardiner's River once vitreous like the obsidian of Tolcsva ? 

 If so, then the Pont-y-Gromlech rock may be a devitrified obsidian. 

 On the other hand we may be justified in assuming that the micro- 

 crystalline condition of the rhyolite from Gardiner's River, and, in- 

 deed, that of any rhyolite, may be an immediate result of cooling, yet 

 identical with the condition which often supervenes when solidified 

 glassy rocks undergo devitrification. 



We may therefore, I think, be allowed to consider the Pont-y- 

 Gromlech rock either a rhyolite or a devitrified obsidian ; for, in the 

 first case, it may be regarded as an obsidian devitrified at its birth ; 

 in the second, as an obsidian devitrified in its old age. The peculiar 

 structure of this rock cannot, however, be reconciled with any pro- 

 cess of crystallization, but must rather be regarded as the result of 

 fluxion in what, at the time, must have been a nearly or quite 

 amorphous magma. 



These considerations lead me to the conclusion that it is unwise 

 either to employ too many names for rocks of the same character, 

 or to give distinctly different names to rocks of different character 

 which may once have been identical. Still there are difficulties in 

 framing a new nomenclature ; for, if we decline to speak of what 

 we believe to be a devitrified obsidian as a rhyolite or a felstone, we 



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