III.] THE CHEMISTRY OF METAMORPHIC ROCKS. 19 



us to form some conception of processes which we're universal in eozoic times, the 

 notion that any of the great series of crystalline rocks are the stratigraphical equiva- 

 lents of formations elsewhere known to us as uncrystalliue sediments, will be found to 

 rest on very uncertain evidence. Those crystalline rocks have doubtless, since their 

 deposition, undergone certain molecular modifications (by what has been named 

 diagenesis) which have changed their original aspect ; but something of the same sort 

 is to a greater or less extent true of many sedimentary rocks to which we do not give 

 the name of metamorphic. This term has not only come to be familiarly used as a 

 synonyme for all crystalline stratified rocks, but is associated with the notion of a 

 profound epigenic change (pseudomorphism) extended alike to uncrystalline sediments 

 and to crystalline eruptive rocks ; a notion has been embodied in the assertion that 

 " regional metamorphism is pseudomorphism on a grand scale." See in this connec- 

 tion Essay XIII. and its Appendix. 



AT a time not very remote in the history of geology, when all 

 crystalline stratified rocks were included under the common des- 

 ignation of primitive, and were supposed to belong to a period 

 anterior to the fossiliferous formations, the lithologist confined 

 his studies to descriptions of the various species of rocks, with- 

 out reference to their stratigraphical or geological distribution. 

 But with the progress of geological science a new problem is 

 presented to his investigation. While paleontology has shown 

 that the fossils of each formation furnish a guide to its age 

 and stratigraphical position, it has been found that sedimentary 

 strata of all ages, up to the tertiary inclusive, may undergo 

 such changes as to obliterate the direct evidences of organic 

 life ; and to give to the sediments the mineralogical characters 

 once assigned to primitive rocks.* The question here arises, 

 whether in the absence of organic remains, or of stratigraphical 

 evidence, there exists any means of determining, even approxi- 

 mately, the geological age of a given series of crystalline strati- 

 fied rocks ; in other words, whether the chemical conditions 

 which have presided over the formation of sedimentary rocks 

 have so far varied in the course of ages, as to impress upon 

 these rocks marked chemical and mineralogical differences. In 

 the case of unaltered sediments it would be difficult to arrive 

 at any solution of this question without greatly multiplied 

 analyses ; but in the same rocks, when altered, the crystalline 

 minerals which are formed, being definite in their composition, 

 and varying with the chemical constitution of the sediments, 



* See the remarks on the preceding page. 



