122 
ELEMENTS OE GEOLOGY. 
In the present work, therefore, the four great classes of 
rocks, the aqueous, plutonic, volcanic, and metamorphic, will 
form four parallel, or nearly parallel, columns in one chron¬ 
ological table. They will be considered as four sets of mon¬ 
uments relating to four contemporaneous, or nearly contem¬ 
poraneous, series of events. I shall endeavor, in a subse¬ 
quent 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 geo¬ 
logical period, and how the earth’s crust may have been con¬ 
tinually remodelled, above and below, by aqueous and igne¬ 
ous 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 volcanic 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 su¬ 
perficial igneous rocks were in progress contemporaneously 
with others of subterranean and plutonic origin, 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, tkat during all 
these changes the solid crust of the earth has been increasing 
in thickness. It has been shown, that so far as aqueous ac¬ 
tion is concerned, the gain by fresh deposits, and the loss by 
denudation, must at each period have been equal (see above, 
p. 96); 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 vol¬ 
canic rocks which they support, I have already 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 an¬ 
swer 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 buildings, or the contrary. So also 
in regard to the relative age of the superior and inferior por¬ 
tions of the earth’s crust; we can not hazard even a conjec¬ 
ture on this point, until we know whether, upon an average, 
the power of water above, or that of heat below, is most effi¬ 
cacious in giving new forms to solid matter. . 
