Prof. Keilhau on the Present State of Geology. 347 



much as chemistry has artificially produced the analogues of 

 some of those minerals of which the problematic mountain 

 rocks are composed, it can without doubt point out with cer- 

 tainty particular modes by which nature may have operated 

 in the formation of such minerals ; but can we depend on 

 chemistry being able to point out all the modes of operation it 

 is possible for Nature to have employed in such cases ? And if 

 this were taken for granted, how can chemistry decide as to 

 which of these was actually employed in the instances in 

 question X 



Chemistry already points out more than one way in which 

 such minerals as are now under consideration may have been 

 produced, so that there may really be more than one mode of 

 formation ; and, as that science is itself only in a state of pro- 

 gression, it is possible that afterwards it may be able to indi- 

 cate other processes besides those already discovered. But 

 perhaps those very modes of formation * to which the rocks in 

 question owe their origin and development may, even in time 

 to come, remain undiscovered ; for chemical knowledge of this 

 kind rests on experiment, and it cannot well be assumed that 

 art will ever have it in her power to apply all the means by 

 which Nature herself has operated in her great laboratory. 



But let us assume that every mode is known by which the 

 minerals composing granite, basalt, crystalline limestone, &c. 

 could have been formed. How, I would ask, can chemistry 

 determine which of the processes Nature has followed, when, 

 in tlie production of these rocks, she effected the formation of 

 the minerals of which they are constituted ? We remarked 

 that the rocks of which we are now speaking had an obscuree 

 origin and dev elopment ; the process by which they becam 

 what we now find them is concealed from direct observation ; 



• Perhaps if we saw the subject in a proper point of view, we shoiild recognise 

 only one mode of formation for crystals. But so long as the ultimate causes are 

 unknown to us, much, in our estimation, seems diflferent, which is really not so. 

 When we find a compact mass gradually becoming crystalline at the ordinary 

 temperature, we naturally regard the crystals produced as formed in a different 

 manner from those crystals which result from fusion. If, again, as is credible, 

 the process of cementation can be caused by diflferent influences, so here also, ao^ 

 cording to our limited observation, several apparently distinct modes of operation 

 may be supposed to have been in action. 



