32 Dr. Mac Culloch o?i the Origin, Materials, 



variations occur in the limestones ; and these, it is obvious, have 

 been subject, in many instances, to chemical laws, as well as to 

 the influence of organized bodies, from which the others have 

 been comparatively exempted. That the secondary strata should 

 contain sandstone and schists, is easily accounted for, by recol- 

 lecting that these must be the result of the destruction of the 

 older rocks ; the more durable mineral remaining distinct, while 

 the compound ones have been reduced to powder. But the 

 question of the formation and origin of all these rocks will be ex- 

 amined more particularly hereafter. 



On the Consolidation of Rocks. 



As almost all the rocks with which we are acquainted have 

 been formed out of our sight, the mode in which the earths, or 

 simple minerals, became consolidated into these forms, is to us a 

 matter of inference from analogy, not of observation. If dis- 

 cussion could have determined this question, it would have been 

 solved long since ; as most of the schemes which have been 

 called Theories of the Earth have been chiefly engaged in this 

 pursuit, and as neither argument nor assumption has been spared 

 in attempting to establish the exclusive views of many of these 

 theorists. To record the terms under which the different par- 

 tisans have thought fit to array themselves, would be to foster 

 and perpetuate an opposition, often arising, more, perhaps, from 

 the colours of the different banners, than from the merits of the 

 cause. Blue and green factions have often exerted their in- 

 fluence beyond the limits of eastern or western empires, and in 

 far other pursuits than politics. 



Fortunately, all rocks have not been formed in the depths of 

 the earth, and fortunately, also, it is in the power of art to 

 produce some of these substances from indiscriminate mixtures 

 of their elements. It is our business to try how far we can 

 extend analogies from the visible to the invisible, from the 

 present to the past. If this process will not carry us far, it is 

 at least the only rational mode of investigation in our power. 



Volcanoes are among the most active and impressive sources 



