FresidenV s Address. 
123 
feet — steam will be generated. This will separate the strata 
a little further, more water will sink down until an expan- 
sive force is formed sufficient to heave up the superincum- 
bent strata, and when steam escapes, either by a volcanic 
vent or into the bottom of the sea, the rapid vibratory 
motion will be produced, which constitutes the earthquake. 
Hot springs are explained on the same principle — only, in 
their case, the water does not arrive at so great a depth 
before it is again returned to the surface. Such, then, is a 
brief summary of the celebrated Huttonian theory of the 
earth, and the principal geological problems which it is 
supposed to explain. 
I have been induced to collect and arrange the foregoing- 
facts and arguments which have been advanced in support 
of the theory of a central heat, chiefly as suggestive, and 
pointing out to us the many and difficult questions that yet 
remain to be solved before a true theory of the earth can be 
hoped for. Setting aside the greater problems, which, in 
the present state of science, are almost beyond our reach, I 
shall mention two lesser ones, but still important as bearing 
directly on the theory, and capable of solution by observa- 
tion and experiment. 
The first is on the Aqueous^ or the Igneous origin of Granite. 
This problem is generally assumed as sufficiently established, 
but it is very far from it ; and the valuable and carefully 
conducted experiments made by Mr Alex. Bryson, which 
are well known to the members of this Society, are directly 
opposed to the view of its igneous origin. 
Secondly, some interesting Experiments on the Metamor- 
phic BocJcs, if not altogether against the theory of igneous 
metamorphism, have proved, in some instances at least, that 
the temperature could not have been very great. The ex- 
periments were made by exposing thin plates of rocks, or 
crystals cut in certain directions, to the slow action of solu- 
tions of acids and alkalies of difl"erent degrees of concentration. 
The result of this action is the gradual removal of some or 
all of the bases, a residue being left, the structure and com- 
position of which indicated the mode of formation of the ori- 
ginal rock. The residue or skeleton represents, for instance, 
VOL. HI. R 
