408 



E. EUTLET ON THE MICKOSCOPIC STRUCTURE OP 



commit ourselves to opinions concerning former lithological condi- 

 tions, of which we sometimes have only very imperfect evidence. 

 Moreover, if we decline to distinguish a rhyolite from an obsidian, 

 we may with equal justice refuse to recognize a trachyte. It appears 

 desirable to follow the classification of some of the continental peno- 

 logists in grouping all the highly-silicated vitreous rocks together 

 under the term "hyaline rhyolite;" and, as our knowledge of this 

 group increases, we may possibly find that its members differ more 

 in structure than in any other respect, and that even many of the 

 structures are common. 



I have already stated that the Pont-y-Gromleeh rock is now, to 

 all intents and purposes, a felsite-schist * or felstone. "Wherein lies 

 the difference between a felstone and a rhyolite, so far as mineral 

 constitution is concerned ? The constituents are essentially felspar 

 and quartz (similar to that in plutonic rocks), the felspar in felstones 

 being chiefly orthoclase, and that in rhyolites the variety sanidine, 

 which occurs only in volcanic rocks. In the majority of cases it is 

 difficult to ascertain with certainty the precise nature of the minute 

 crystalline granules of felspar in the ground-mass either of a fel- 

 stone or of a rhyolite. The rhyolites commonly contain more or less 

 vitreous matter ; but in the older examples this, if it existed, has 

 since undergone devitrification ; and the product is a microcrystalline 

 aggregate which cannot be distinguished from the rest of the rock, 

 except perhaps in some instances by difference in texture. The 

 difference therefore between a rhyolite and a felstone is mineralogi- 

 cally a very small one. The quartz-rhyolites or liparites are closely 

 related to quartz-porphyry ; so that here, again, we have a mineralo- 

 gical affinity in rocks which are, on the one hand, volcanic, and, on 

 the other, plutonic; and in many cases, especially among the older 

 rocks, it is by structure alone that they can be distinguished. The 

 quartz-porphyries may be regarded as spurs or dykes emanating 

 from granitic masses ; these dykes, like the margins of the granitic 

 masses, are commonly poor in mica ; or that mineral may be totally 

 absent. The dykes, again, like the margins of the granitic masses, 

 3?re usually fine-grained. We have, then, a tolerably well-defined 

 series, consisting of : — 



Bhyolitic series... t^lf™ rhyoKteB (obsidian pitchstone, perlite, 



J [ &c), quartz-rhyohte, and trachyte. 



Granitic series . . . Quartz-porphyry, and granite. 



Trachyte bears much the same structural relation to quartz-rhyolite 

 that granite does to quartz-porphyry. Bathymetrical conditions pre- 

 clude the granitic series from having any vitreous representatives. 

 That there is a passage from the granitic to the rhyolitic series, as 

 suggested by many petrologists, seems more than probable. So far 



* It is here worthy of remark that Daubree has succeeded in superinducing 

 schistose as well as fibrous and spherulitic structures in glass tubing, by heat- 

 ing it in presence of water up to a temperature of about 400° C, and under a 

 pressure which he estimates at more than 1000 atmospheres (Etudes Synthe- 

 tiaues de Geologie Experimentale, p. 156 et seq.). 



