224? PREVAILING THEORY CONCERNING [Ch. XI. 



place, and that a more crystalline texture should ensue. It 

 may, therefore, be easily supposed that all traces of shells, 

 and other organic remains, may be destroyed, and that new 

 chemical combinations may arise, without the mass being so 

 fused as to wholly obliterate the lines of stratification. 



According to these views, gneiss and mica-schist may be 

 nothing more than micaceous and argillaceous sandstones 

 altered by heat ; and certainly, in their mode of stratification 

 and lamination, they correspond most exactly. Granular 

 quartz may have been derived from siliceous sandstone, com- 

 pact quartz from the same : clay-slate may be altered shale, 

 and shale appears to be clay which has been subjected to 

 great pressure. Granular marble has probably originated in 

 the form of ordinary limestone, having, in many instances, 

 been replete with shells and corals now obliterated, while 

 calcareous sands and marls have been changed into impure 

 crystalline limestones. So that all the primary strata, so 

 beautifully compact and crystalline as they now are, have 

 once been in the state of ordinary mud, clay, marl, sand, 

 gravel, and other deposits now forming by aqueous agency. 



Before concluding this abridgment of Ly ell's exposition of 

 the igneous theory, as applicable to the primary rocks, it must 

 not be omitted to mention, that he proposes the term hypo- 

 gene as a substitute for that of primary, a word implying 

 the theory that granite and gneiss are both nether-formed 

 rocks, or rocks which have not assumed their present form 

 and structure at the surface : and to call the unstratified 

 primary rocks Plutonic, and the stratified metamorphic, in- 

 dicative of the changes which they are supposed to have 

 undergone. 



These terms are certainly in perfect accordance with the 

 theory : and, if we could be assured that this theory is as 

 securely established as are the fundamental principles of as- 

 tronomy, they might be adopted without inconvenience : but 

 geology has been, and is still, subject to a constant fluctuation 

 of opinions ; and has been already too much perplexed by a 



