TO STRATIFIED ROCKS. 41 
which the common consent of nearly all modern 
geologists and chemists refers to the action of 
fire. The agency of central heat, and the ad- 
mission of water to the metalloid bases of the 
earths and alkalies, offer two causes which, taken 
singly or conjointly, seem to explain the pro- 
duction and state of the mineral ingredients of 
these rocks ; and to account for many of the 
grand mechanical movements that have affected 
the crust of the globe. 
The gradations are innumerable, which con- 
nect the infinite varieties of granite, syenite, 
porphyry, greenstone, and basalt with the tra- 
chytic porphyries and lavas that are at this day 
ejected by volcanoes. Although there still re- 
main some difficulties to be explained, there is 
little doubt that the fluid condition in which all 
un stratified crystalline rocks originally existed, 
was owing to the solvent power of heat ; a power 
whose effect in melting the most solid materials 
of the earth we witness in the fusion of the 
hardest metals, and of the flinty materials of 
glass.* 
* The experiments of Mr. Gregory Watt on bodies cooled 
slowly after fusion ; and of Sir James Hall, on reproducing arti- 
ficial crystalline rocks, from the pounded ingredients of the 
same rocks highly heated under strong pressure ; and the more 
recent experiments of Professor Mitscherlich, on the production 
of artificial crystals, by fusion of definite proportions of their com- 
ponent elements, have removed many of the objections, which 
were once urged against the igneous origin of crystalline rocks. 
