456 



Transactions . — Geo logy , 



One of the most interesting illustrations of the change produced by 

 hydrothermal agency has been described by Professor Daubree, who found 

 that the water of the springs of Plombieres, in the Vosges, which have a 

 temperature of 160° Fahr., had formed zeolites in the concrete of the Eoman 

 aqueduct built for conveying the water, the concrete being composed of hme, 

 fragments of brick and sandstone. The minerals found include apophyllite, 

 chabazite, and opal. 



In the following table the relations of the various eruptive rocks forming 

 dykes, lavas, and scoria are shown : — 



— 



Compact. 



Crystalline, gi-aniilar. 



Glassy, scoriaceous. 



[Old 

 Acidic \ 



[ Modern 



Inter 



(Old .. 

 Basic - 



( Modern 



Felstoue 

 Trachyte 

 mediate Trachy- 

 Aphanite 

 Basalt 



Quartziferous Porphyry, 



Elvan 

 Trachyte Porphyry . . 



dolerite. 



Diorite 



Dolerite 



Pitchstone, Perlite 



Obsidian, Pitchstone, 

 Perhte, Pumice 



Tachylite 

 Tachylite, Pumice 



The characteristic ingredients of these leading varieties may be stated 

 thus : — felstones have orthoclase felspar and quartz, the glassy conditions 

 are pitchstones and perlites. Trachytes, their modern representatives, are 

 composed almost wholly of a confused mass of crystals of sanidiu without 

 perceptible free quartz ; they are often porphyritic, the glassy form is 

 obsidian. Hornblende is fi'equently x^i'esent in these acidic rocks. The 

 diorites comprise the hornblendic basic rock with orthoclase and oligoclase 

 felspars. The dolerites, their modern representatives, have augite with 

 sanidin and Labradorite felspars, tachylite forming the glassy condition ; 

 it closely resembles obsidian. The ashy and tufaceous kinds are found con- 

 solidated into felstones and aphanite-slates ; microscopic examination shows 

 these slates to contain crystals with fused surfaces, or with vitreous coatings, 

 and isolated shreds of glassy matter in strings or bands (Eutley, Q.J.G.S., 

 vol. XXXV., p. 338.) 



The glassy varieties have been formed by rapid cooling of the molten 

 mass, for when basaltic rocks have been experimentally melted, and 

 cooled slowly, a state very similar to the original has been attained ; but 

 when cooled rapidly, they have assumed a dark brittle glassy condition, re- 

 sembling obsidian. The perfectly amorphous condition of common glass is 

 seldom attained in the natural rock, minute crystals of pyroxene and felspar 

 being generally more or less scattered through the glassy matrix. The 

 glassy condition being a particularly unstable one, the obsidians and ro'cks 

 with allied glassy structures like perlite have been altered into pitchstones. 



