AND ANALOGIES, OF ROCKS. 197 



while the compound ones have been reduced to pow- 

 der. That they alternate, is the consequence, partly 

 of distinctions in the deposited materials, partly of 

 the hydrostatic actions by which substances unequally 

 gravitating in water are separated, and partly, of 

 animal growth and reproduction. 



On the Consolidation of Rocks. 



As almost all the rocks have been formed out of 

 our sight, the mode in which the earths, or simple 

 minerals, have become consolidated into these forms, 

 is to us a matter of inference from analogy ; not of 

 observation. If discussion could have determined 

 this question, it would have been solved long since ; 

 as most of the schemes which have been called Theo- 

 ries 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 partizans 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. 



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 im- 



