ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 477 



ino-s and direct our observations ; and without them wc cannot 

 proceed a step on firm ground. They distinguish the philoso- 

 pher from the empiric, and combine scattered observations into 

 a body of useful and rational science. Even in the science of 

 nature, as in that of numbers, the assumption of imaginary or 

 erroneous laws, leads to the discovery of the truth. The history 

 of astronomy is in itself a lesson to those who ignorantly un- 

 dervalue the pursuit of general laws. Bewildered in spheres and 

 vortices, it arose, as in a moment, complete, from the theory of 

 gravitation. 



" Hence the consideration of secondary causes, forms, not only 

 a legitimate, but an essential part of geological science. That 

 science, like all others, comprises the history of all the facts which 

 it involves ; and from these, it establishes certain general analo- 

 gies.- Ascending a step higher it declares the laws which have 

 regulated, and will continue tp regulate, all the phenomena of the 

 globe; and thus finally establishes a legitimate theory of the 

 earth." 



No trace of organic remains has been discovered in these mica- 

 ceous, chloritic or argillaceous slates, nor even in the limestones 

 associated with them. The adherents of the metamorphic 

 hypothesis attempt to account for this by supposing that the fos- 

 sils have been obliterated by the agencies which have effected the 

 alteration. But even in the greywacke slates and sandstones, 

 traces of life are rare ; and it is only in the very newest strata of 

 that' series, that they become at all frequent, and then they 

 belong to the inferior grades of animal organisms. That the air- 

 breathers, recently described by Dr. Dawson, first make their 

 appearance in the coal-measures, may be regarded as a proof of 

 the absence of free oxygen from the atmosphere which existed 

 dnring the deposition of the Lower Silurian rocks. Not until the 

 carbonic acid was to a great extent removed from the atmosphere 

 by the luxuriant vegetation of the coal period, and its place taken 

 up by oxygen, was it possible for air-breathers to exist. The 

 extraordinarily rich vegetation of that epoch was no doubt stimula- 

 ted by the immense quantities of carbonic acid in the atmosphere, 

 and the exceedingly warm climate which then prevailed over the 

 whole surface of the earth. This warm climate, we are justified 

 In supposing, was caused more by the radiation of heat from the 

 interior of ihe earth, than by solar influence. So that it is possi- 

 ble to trace a connection between the phenomena of internal heat 



