454 Transactions.—Geology. 
When granite and other rocks have penetrated superincumbent strata, 
the contact of the molten mass has usually produced a change in the neigh- 
bouring rocks, but of a different character to that widespread uniform 
character described under metamorphism. The difference has been brought 
about through its sudden character, and probably by the loss of a large 
portion of watery vapour, causing a vitrifying effect to be produced. 
Hornblende-slates are frequently formed along the contact margins of 
granite and clay-slates. (Q.J.G.S., vol. xxxii., p. 187, J. A. Phillips ; 
Q.J.G.8., vol. xxxiv., p. 488). 
Metalliferous veins are usually found to have been formed by the 
occurrence of intrusive rocks in the vicinity, the latter having been usually 
decomposed by acids and vapours, the introduced metals, or the metals 
from them, being deposited in veins. 
Mr. J. A. Phillips in Q.J.G.S., vol. xxxv., p. 391, describes the dis- 
trict of Steamboat Springs, Nevada, where “ fissures are being lined with 
siliceous incrustations which are being constantly deposited, while a central 
longitudinal opening allows the escape of gases, steam and boiling water ; 
the water is slightly alkaline and contains carbonate of sodium, sulphate of 
sodium, common salt, etc.” These springs have deposited cinnabar 
(ore of mercury) with the silica (both amorphous and crystalline, the latter 
containing the usual liquid cavities and ordinary optical and other charac- 
ters of ordinary quartz). At other springs in the same district silver and 
gold have been found enclosed in sinter-like deposits. In Australia gold 
occurs in pyrites contained in diorites and granite, and gold mines are 
worked in these rocks. Mineral veins often show by their structure that 
the fissures (Q.J.G.S., vol. xxxii., p. 169) they fill have been widened a: 
peatedly, probably by the force of crystallization, successive infiltration 
having filled the fissure with siliceous and other substances forming a banded 
structure. The metals when they occur may either have been deposited 
from solution or by sublimation. The tin-bearing bands of schorl rock 2 
granite of Cornwall, have been proved to have been formed through aa 
decomposition of the granite along the sides of leaders or veins. Granites 
vary from coarsely porphyritic granites to the fine grained elvans (quartz 
ferous porphyry) in which mica is present. The porphyritic texture 1 due 
to the inequality in the erystallizing power of the various minerals, felspat 
and mica crystallizing Jily than quartz, the latter always ocewri0S 
in consequence in more irregular forms than the former. 
Hornblende and schorl are sometimes found replacing mica to a great a 
extent, forming syenite and schorlaceous-granite, and when only @ 
