378 RECAPITULATION OF THE OBJECTIONS [Oh. XVI. 



and throws of veins appear at first sight to confirm the me- 

 chanical origin of these bodies, yet they will not stand the 

 test of a rigid scrutiny ; for of whatever kind the veins may be, 

 when several are traversed by one vein, the extent of the 

 apparent movement is in general not uniform, and sometimes 

 varies in the case of each individual vein : and it may be 

 likewise added, that similar phenomena occur not only in 

 veins, but also in the strata and laminae of rocks, when no 

 cross vein, or even fissure, can be detected. 



Finally, we have advanced objections to the theory concern- 

 ing the metamorphosis of sedimentary deposits into crystalline 

 or primary slates. 1. We have expressed some doubts whether 

 the transitions which have been observed between these rocks, 

 can be regarded as a criterion of identity ; because, on this 

 principle, the secondary and tertiary formations might be 

 referred to the same epoch, and even granite, gneiss, and 

 similar rocks might be confounded with adjacent conglomerates 

 which are composed of their detritus : and it has been 

 attempted to illustrate the fallacy of such transitions being 

 considered as evidence of coeval formation, by the occurrence 

 of beds of transported debris on the shores of Mount's Bay, 

 which may, by such means, although overlying sand containing 

 sea-shells, and a stratum of decayed wood or a submarine 

 forest, be proved to be identical with the primary slates ; or, in 

 other words, the latter would be demonstrated to be the 

 metamorphic strata of sedimentary deposits, belonging to the 

 modern geological epoch. 2. The facts by which the 

 advocates of the Plutonic theory have attempted to prove the 

 power of fire to convert secondary strata into primary, have 

 been passed in review ; the effects of trap-rocks on sedimentary 

 deposits have been compared therewith ; and then we have 

 attempted to show, by referring to chemical analyses, that, with 

 the exception of dolomite, and perhaps of porcelain-jasper and 

 of analcime, the nature of which seems to be doubtful, all 

 these changes are such as may be attributed to a mechanical 

 alteration of the structure of these rocks, resulting from a 



