Ch. XL] THE PRIMARY ROCKS. 225 



similar terminology ; it is therefore desirable that, in future, 

 the cultivators of this science should avoid, as much as pos- 

 sible, the introduction of names which have a theoretic signi- 

 fication. For this reason, we have taken the liberty of 

 omitting the terms hypogene and metamorphic, in the following 

 observations of Lyell on the relative age of the primary rocks. 



" It it undoubtedly true," continues our author, " that we 

 can rarely point out primary stratified or unstratified rocks, 

 which can be proved to have been formed in any secondary 

 or tertiary period. We can, in some instances, detect granites 

 of posterior origin to certain secondary strata; and that 

 secondary strata have sometimes been converted into gneiss, 

 mica-slate, and others resembling primary. But examples of 

 such phenomena are rare : and their rarity is quite consistent 

 with the theory, that the primary rocks, both stratified and 

 unstratified, have been always generated in equal quantities 

 during periods of equal duration. We conceive that the 

 granite and gneiss, formed more recently than the carboni- 

 ferous era, are for the most part concealed ; and those por- 

 tions which are visible can rarely be shown, by geological- 

 evidence, to have originated during secondary periods." 



" A considerable source of difficulty and misapprehension, 

 in regard to the antiquity of the primary strata, may arise 

 from the circumstance of their having been deposited at one 

 period, and having assumed their crystalline texture at 

 another. As the progress of decay and reproduction, by 

 aqueous agency, is incessant on the surface of the continents 

 and in the bed of the ocean, while the primary rocks are 

 generated below, or are rising gradually from the volcanic 

 foci, there must ever be a remodelling of the earth's surface 

 in the time intermediate between the origin of each set of 

 primary rocks and the protrusion of the same into the at- 

 mosphere or the ocean. The time required for so great a 

 developement of subterranean elevatory movements, might 

 well be protracted until a deposition of a series of sedimen- 

 tary rocks, equal in extent to all our secondary and tertiary 



Q 



