Ch. viii] of kocks in general. 95 



shown, that so far as aqueous action is concerned, the gain by fresh deposits, 

 and the loss by denudation, must at each period have been equal (see above, 

 p. 68) : 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 

 counterbalanced the loss sustained by the melting of materials previously 

 consolidated. As to the relative antiquity of the crystalline foundations of 

 the earth's crust, when compared to the fossiliferous and volcanic rocks 

 which they support, I have already stated, in the first chapter, that to pro- 

 nounce an opinion on this matter is as difficult as at once to decide w^hich 

 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 answ^er 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, un- 

 til we know whether, upon an average, the power of water above, or that 

 of heat below, is most efficacious in giving new forms to solid matter. 



After the observations which have now been made, the reader will per- 

 ceive that the term primary must either be entirely renounced, or, if re- 

 tained, must be differently defined, and not made to designate a set of 

 crystalline rocks, some of which are already ascertained to be newer than 

 all 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. To prevent con- 

 fusion, I shall sometimes speak of these last as the primary fossiliferous 

 formations, because the word primary has hitherto been most generally 

 connected with the idea of a non-fossiliferous rock. Some geologists, to 

 avoid misapprehension, have introduced the term Paleozoic for primary, 

 from 'jfaXam, " ancient," and ^wov, " an organic being," still retaining the 

 terms secondary and tertiary ; Mr. Phillips, for the sake of uniformity, has 

 proposed Mesozoic, for secondary, from (xerfocr, " middle," &c. ; and Caino- 

 zoic, for tertiary, from xajvo^, " recent," &c. ; but the terms primary, sec- 

 ondary, and tertiary are synonymous, and have the claim of priority in 

 their favor. 



If we can prove anj- plutonic, volcanic, or metamorphic rocks to be 

 older than the secondary formations, such rocks will also be primary, ac- 

 cording to this system. Mr. Boue, having with propriety excluded the 

 metamorphic rocks, as a class, from the primary formations, proposed to 

 call them all " crystalline schists." 



As there are secondary fossiliferous strata, so we shall find that there 

 are plutonic, volcanic, and metamorphic rocks of contemporaneous origin, 

 which I shall also term secondary. 



In the next chapter it will be shown that the strata above the chalk 

 have been called tertiary. If, therefore, we discover any volcanic, plutonic, 

 or metamorphic rock§, which have originated since the deposition of the 

 chalk, these also will rank as tertiary formations. 



