296 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. 



candidly receive any suggestions which may lead either to 

 the subversion, or to the firmer establishment of the theory 

 which they advocate. 



It is proposed, in the first place, to examine the nature of 

 the transition or passage which occurs among the secondary 

 rocks, and between these and the primary slates ; and ascer- 

 tain whether the evidence thus furnished is conclusive, that 

 all rocks so connected have had the same origin, and belong 

 to formations of the same epoch. 



In the primary rocks, transitions appear to be effected in 

 two ways : in the one, the crystalline constituents, when the 

 rocks are compound, or the component particles, when sim- 

 ple, gradually pass from large and coarse concretions to par- 

 ticles so minute as not to be detected by the eye ; and during 

 this change, rocks of very various aspects are produced, which 

 are farther varied by a difference in the proportion of their 

 component parts : this transition is very similar to that which 

 commonly obtains amongst the secondary rocks, owing to the 

 intermixture of two deposits ; either in consequence of the 

 sediment following the laws of gravity during its accumula- 

 tion, or to the gradual change of circumstances under which 

 they have been produced. In the other case, however, the 

 transition between different primary rocks is accomplished by 

 a gradual change, not only in the proportions, but also in the 

 mineralogical characters of the constituent parts. Thus, 

 the felspar and quartz of granite pass into granular and 

 compact felspars, that is, into eurite and felsparite; mica, 

 by degrees, assumes the properties of talc, shorl of horn- 

 blende, hornblende of diallage, or of chlorite ; in short, many 

 other transformations well known to geologists, and which, of 

 course, are accompanied by a corresponding difference in the 

 nature of the rocks. The latter may be called a mineral ; 

 the former, a mechanical transition. 



When crystalline rocks are found to pass into each other 

 by mineral transitions, it may be presumed that they do 

 belong to the same formation : but when the passage is 



