249 
will come in on the temperature and pressure of rock masses 
which will finish this sub-division. There is no means 
of estimating * the extreme slowness with which the changes in 
them have been brought about, except such as are suggested by 
the changes of level in land that have been observed to be now 
in progress; and although high temperatures may be necessary 
to approximate to similar conditions in our experiments, it is 
probable that in the lengthened periods over which the natural 
operations extended, the heat involved may have been less than 
would at first have been expected, and it is more than probable 
that the phenomena are not to be explained by the action of 
heat alone. All experiments at the earth’s surface are necessar- 
_ily under a pressure which is infinitesimal in comparison with 
that of the superincumbent rock, which has since been denuded 
from a granitic district, sometimes for a thickness of miles, to 
say nothing of the force of pressure from lateral contraction 
which is superadded. And while the water within the rock at 
once escapes in a furnace experiment, the water is inevitably 
imprisoned in a metamorphosed rock, so that the conditions of 
the experiments are not the same. The presence of water 
appears to be necessary to the production of such crystalline 
forms for minerals as are met with in nature, for in blast furnace 
slags which are run out at a temperature of about 3700° F., 
only complex feathery skeletons of crystals are commonly 
formed with belonites and trichites scattered in the glass. And 
when igneous rocks, such as basalt, are artificially melted, the 
augite crystallises in flat feathery plates, like those of furnace 
slags, which are rarely if ever seen in nature; and the felspar 
prisms end in complex fan-shaped brushes, so that the structure 
of the rock is changed by the conditions of liquefaction and 
consolidation. Similarly when the Leicestershire syenite is 
fused and slowly cooled, the solid crystals are lost and replaced 
by feathery skeleton crystals of magnetite, and flat prisms of 
triclinic felspar ending in fan-shaped brushes. 
As to the cavities in granites the same authority remarks, 
“the proof of the operation of water is quite as strong as that 
* Sorby Q. J . Geol. Soc. vol. xxxvi. Address, p. 73. 
