26 T. S. Hunt on Voleanie Action. 
great as now, or one degree for each three feet in depth (Geol. 
Journal, viii, 59.) The conclusion is inevitable that a a 
of things must have existed du uring long periods in the 
of the cooling globe, when the accumulation of aparativaly 
thin layers of sediment would have been sufficient to give rise 
to all the phenomena of metamorphism, vulcanicity, and move- 
ments ‘of the crust, whose origin Herschel has so well ex- 
laine 
Coming, i in the next place, to consider the influence of pres- 
sure upon the buried materials derived from the mechanical and 
chemical disintegration of the primitive crust, we find that by 
the presence of heated water throughout them, they are placed 
While pressure raises the fusing point of such bodies as expand ? 
im passing into the liquid state, it depresses that point for those 
which, like ice, contract in becoming liquid. The same princi- 
ple extends to that liquefaction which constitutes solution ; 
where, as is with few exceptions the case, the process is attended 
with condensation or diminution of volume , pressure will, as 
shown by the experiments of Sorby, augment the solvent power 
of the liquid.* Under the influence of the elevated tempera- 3 
ture and the great nit which prevail at considerable depths, | 
sediments should, therefore, by the effect of the water which 
they contain, acquire a a certain degree of liquidity, rendering not 
seme the suggestion of Scheerer, that the presence of five 
ent of water may suffice, at gs et Ma oo eo 
Se to give to a granitic mass a hq g at 
once of the character of an igneous and an aqueous oat chm 
studies by Mr. Sorby of the cavities in apeals have led him to 
conclude that the constituents of granitic and trachytie rocks 
have crystallized in the presence of liquid water, under great 
pressure, at tem om hate not above redness, and ‘consequently 
that required for ce igneous fusion. ' The 
re ai admitted.’ In this connection, the reader is referred 
to the Geological Magazine for February, 1868, page 57, where 
the history of this question is cceccion § 
__ It may here be remarked that if we regard the liquefaction 
of heated rocks under great pressure, and in presence of water, 
asa eee. - solution rather than of fusion, it would follow 
that dimin of pressure, as supposed. b Mr. a: would 
cause, not Se ection, but the reverse. The mechanical pres- 
sure of great accumulations of sediment is to be regarded as co- 
rms with heat to augment the solvent action of the water, 
~ geeks — Lecture, Royal Society, 1863. 
