President's Address. 
121 
quantity of heat, which it would carry upwards with it until 
it lost it by radiation, when it would again become con- 
densed and fall in rain as before. As the crust cooled, its 
internal capacity would become diminished by contraction, 
and as it could not compress the fluid contents into a smaller 
space, it would itself give way, and large rents and fissures 
would ensue. Through these rents water could easily 
descend to the melted matter beneath, and the consequence 
would be the formation of a prodigious quantity of steam 
under a very high temperature and pressure. This would 
produce further disruption and upheavings of the crust, 
while the rents becoming filled up by the fluid matter from 
beneath, might, in some instances, overflow the consolidated 
strata, and by cooling more rapidly, would assume a different 
mechanical structure and constitution from that of the 
already formed crust. Volcanoes may have originated about 
this time, and have been the means of lessening the efl'ects 
of subsequent occurrences of a like kind, by affording a more 
ready escape to the vapours afterwards generated. These, 
however, would still be produced, in consequence of the suc- 
cessive contractions of the earth's crust, and would give rise 
to elevations and depressions that are observed to have taken 
place at the surface of the earth. This may be supposed 
to have constituted the primitive period, at the conclusion 
of which organised beings were called into existence. The 
nebular hypothesis, although supposed to give considerable 
support to the Huttonian theory, must be carefully distin- 
guished from the Huttonian theory itself The application 
of the theory of a central heat to explain certain geological 
phenomena may now be briefly stated. 
1. The Thickness of the Earth's Crust.—- As the cooling of 
the earth's crust, according to the theory, is a progressive 
change, there was a time when it was much thinner than 
at present, and during these early periods changes from 
contraction must have been very frequent, for every new 
layer as it cooled beneath would contract and press upon 
the fluid part below, and in the efl'ort would be rent in many 
places. It appears as if several of these rents were still in 
existence. There seems to be a great one passing from the 
