212 Dr. Mac Culloch on the Origin, Materials, 



were the produce chiefly of the argillaceous substances, slate or 

 shale. The more particular evidences in confirmation of this 

 opinion, belong to the histories of trap and granite, on which I 

 cannot here enter. 



Of Transitions among Rocks, 



The last question respecting rocks that appears to require ex- 

 amination, relates to the transitions, real or imaginary, that take 

 place between different kinds of rocks. Being formed, as we have 

 seen, of so few substances, and possessing so many analogies 

 among each other, such transitions ought to be expected. That 

 they exist, is no reason for an hypothesis which has been main- 

 tained on this subject. 



Because there is a gradation of a certain kind among gneiss, 

 micaceous schist, and quartz rock, and because it is possible, by 

 selecting particular specimens, to make that transition still more 

 extensive, it has been argued that all these rocks originated at 

 one time, from a common solution, and are, therefore, the results 

 of a continued crystallization from a fluid gradually varying. 

 Geologists who have chosen to maintain this doctrine have cer- 

 tainly derived from it great convenience ; inasmuch as they have 

 dispensed with the labour of investigating the diff'erences of these 

 rocks, or describing their characters and connexions. I know 

 not what advantages are to be gained by thus restoring geology 

 to its original chaos ; and as the question of watery crystallization 

 has been sufficiently considered by numerous authors, the reader 

 and writer may both equally be saved the trouble of an uuneces- 

 sary discussion. Such transitions as do actually occur, may 

 easily be accounted for in various ways. In the older strata they 

 may arise from proximity of position in rocks that have been in 

 a state of semi- fusion, and that were formed of similar materials. 

 Thus they are common between gneiss, micaceous schist, and 

 quartz rock, accordingly as these approximate. Thus, by inter- 

 mixture, occasional transitions may also happen between coarse 

 argillaceous schist and quartz rock, or between the fine and gneiss. 

 But these are rare and easily explained; nor is there any tran- 



