PART II. CHAPTER XII. 157 



, Chronological Arrangement of the four Classes of Rocks. 



with others of subterranean and plutonic origin, and some sedi- 

 mentary strata were exposed to heat and made to assume a 

 crystalline or metamorphic structure. 



It can by no means be taken for granted, that during all these 

 changes the solid crust of the earth has been increasing in 

 thickness. It has been shown, that so far as aqueous action is 

 concerned, the gain by fresh deposits, and the loss by denuda- 

 tion, must at each period have been equal ; and in like manner, 

 in the inferior portion of the earth's crust, the acquisition of new 

 crystalline rocks, at each successive era, may merely have coun- 

 terbalanced the loss sustained by the melting of materials pre- 

 viously consolidated. As to the relative antiquity of the crys- 

 talline foundations of the earth's crust, when compared to the 

 fossiliferous and volcanic rocks which they support, I have al- 

 ready stated, in the first chapter, that to pronounce an opinion 

 on this matter is as difficult as at once to decide which of the 

 two, whether the foundations or superstructure of an ancient city 

 built on wooden piles may be the oldest. We have seen that to 

 answer this question, we must first be prepared to say whether 

 the work of decay and restoration had gone on most rapidly 

 above or below, whether the average duration of the piles has 

 exceeded that of the stone buildings, or the contrary. So also 

 in regard to the relative age of the superior and inferior portions 

 of the earth's crust ; we cannot hazard even a conjecture on this 

 point, until we know whether, upon an average, the power of 

 water above, or that of fire below, is most efficacious in giving 

 new forms to solid matter. 



After the observations which have now been made, the reader 

 will perceive that the term primary must either be entirely re- 

 nounced, or, if retained, must be differently defined, and not 

 made to designate a set of crystalline rocks, some of which 

 may be newer than the secondary formations. In this work I 

 shall follow most nearly the method proposed by Mr. Boue, who 

 has called all fossiliferous rocks older than the secondary by the 

 name of primary, which thus becomes a substitute for the term 

 transition, so far as regards the aqueous strata. To prevent 

 confusion, however, I shall always speak of these as the primary 

 fossiliferous formations, because the word primary has hitherto 

 been almost inseparably connected with the idea of a non-fossili- 

 ferous rock. 



If we can prove any plutonic, volcanic, or metamorphic rocks 

 to be older than the secondary formations, such rocks will also 

 be primary, according to this system. Mr. Boue having with great 

 propriety excluded the metamorphic rocks, as a class, from the 

 primary formations, proposed to call them all "crystalline 



