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 effects 

 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 effort 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 



