44 VESTIGES OF THE 



time marked their situations by the extrusion of traps 

 and basalts from below — namely, rocks composed of the 

 crystalline matter fused by intense heat, and developed 

 on the surface in various conditions, according to the 

 particular circumstances under which it was sent up ; 

 some, for example, being thrown up under water, and 

 sojne in the open air^ which contingencies are found to 

 have made considerable difference in its texture and 

 appearance. The gi-eat stores of subterranean heat also 

 served an important purpose in the formation of the 

 aqueous rocks. These i-ocks might, according to Sir 

 John Herschel, become subject to heat in the following 

 manner: — While the surface of a particular mass of rock 

 forms the bed of the sea, the heat is kept at a certain 

 distance from that surface by the contact of the water; 

 philosophically speaking, the mass radiates away the 

 heat into the sea, and (to resort to common language) 

 is cooled a good way down. But when new sediment 

 settles at the bottom of that sea, the heat rises up to 

 what was formerly the surface; and when a second 

 quantity of sediment is laid down, it continues to rise 

 through the first of the deposits, wdiich then becomes 

 subjected to those changes which heat is calculated to 

 produce. This process is precisely the same as that of 

 putting additional coats upon our own bodies; when, 

 of course, the internal heat rises through each coat in 

 succession, and the third (supposing there is a fourth 

 above it) becomes as warm as perhaps the hrst originally 

 was. 



In speaking of sedimentary rocks, we may be said to 

 be anticipating. It is necessaiy, first, to show how such 

 rocks were formed, or how stratification commenced. 



Geology tells us as plainly as possible, that the 

 original crystalline mass was not a perfectly smooth ball, 



