90 CHRONOLOGICAL ARRANGEMENT [Ch. VIII. 



tured, before another body of granitic matter Avas injected into it, or through 

 it, in the form of veins. Some granites are more ancient than any known 

 fossiliferous rocks ; others are of secondary ; and some, such as that of 

 Mont Blanc and part of the central axis of the Alps, of tertiary origin. 

 In short, the universal fluidity of the crystalline foundations of the earth's 

 crust, can only be understood, in the same sense as the universality of the 

 ancient ocean. All the land has been under water, but not all at one 

 time ; so all the subterranean unstratified rocks to which man can obtain 

 access have been melted, but not simultaneously. 



In the present work the four great classes of rocks, the aqueous, plutonic, 

 volcanic, and metamorphic, will form four parallel, or nearly parallel, col- 

 umns in one chronological table. They will be considered as four sets of 

 monuments relating to four contemporaneous, or nearly contemporaneous, 

 series of events. I shall endeavor, in a subsequent chapter on the plutonic 

 rocks, to explain the manner in which certain masses belonging to each 

 of the four classes of rocks may have originated simultaneously at every 

 geological period, and how the earth's crust may have been continually 

 modelled, above and below, by aqueous and igneous causes, from times 

 indefinitely remote. In the same manner as aqueous and fossiliferous 

 strata, are now formed in certain seas or lakes, while in other places vol- 

 canic rocks break out at the surface, and are connected with reservoirs of 

 melted matter at vast depths in the bowels of the earth, — so, at every 

 era of the past, fossiliferous deposits and superficial igneous rocks were in 

 progress contemporaneously with others of subterranean and plutonic ori- 

 gin, and some sedimentary strata were exposed to heat and made to as- 

 sume 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 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 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, 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- 



