456 Transactions.— Geology. 
One of the most interesting illustrations of the change produced by 
hydrothermal agency has been described by Professor Daubrée, who found 
that the water of the springs of Plombiéres, in the Vosges, which have a 
temperature of 160° Fahr., had formed zeolites in the concrete of the Roman 
aqueduct built for conveying the water, the concrete being composed of lime, 
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, granular. Glassy, scoriaceous. 
| Old .. | Felstone - Quartziferous Porphyry, Pitchstone, Perlite 
| Acidic Elvan iw 5 
Modern | Trachyte ..| Trachyte Porphyry .. Uae osm 
erlite, Fumi 
Inter|mediate Trachy-|dolerite. 
aa Old ..| Aphanite ..| Diorite we .. | Tachylite 
Modern | Basalt ..| Dolerite .. —_.. | Tachylite, Pumice 
fo Si oe 
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 sanidin without 
perceptible free quartz; they are often porphyritic, the glassy form 18 
obsidian. Hornblende is frequently present in these acidic rocks. The 
diorites comprise the hornblendic basic rock with orthoclase and oligoclas® 
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 anew 
these slates to contain crystals with fused surfaces, or with vitreous coatings, 
and isolated shreds of glassy matter in strings or bands (Rutley, QI.G5-, 
vol. xxxv., p. 338.) 
The glassy varieties have been formed by rapid cooling of the molten 
