Sentence:  But when we come to look upon them [strata of the earth], not, indeed, as the "prima mobilia", but at least as the instruments, the levers and wedges with which the real <WSD>"primum mobile</WSD>", the central heat, does its rough work on the crust of our globe, they lose their claim to this ideal permanence, and come to be considered as, in fact, "newer" than the rocks they penetrate and displace. 
Meaning of the word marked with <WSD> tags:  The person or thing that is the main impetus for some action; a driving force.