452 A POPULAR EXPOSITION OF THE 
ft is now universally admitted, on proofs the most unanswerable, 
that the various sedimentary and other rocks which make up the solid 
portion of our globe, were not formed during one brief or unbroken 
period, but were gradually elaborated and built up during a long series 
of ages. In areas of very limited extent, for example, even in the 
same cliff-face, or in excavations of moderate depth, we often find 
alternations of sandstones, limestones, clays, &c., lying one above 
another, and thus revealing the fact that the physical conditions pre- 
vailing around the spot in question must have been subjected to 
repeated changes. The same thing is also proved by alternations of 
marine and fresh-water strata in particular localities ; and of deep- 
sea and shallow-sea deposits, in others. Again, the sedimentary rocks 
are frequently found in unconformable stratification, as explained 
above : horizontal beds resting upon the sloping surface or upturned 
edges of inclined strata. (See fig. 60.) Here it is evident that the 
inclined beds must have been consolidated and thrown into their 
inclined positions before the deposition of the horizontal beds which 
rest upon them. In the absence of particular sets of strata in special 
localities, proving extensive denudation or long-continued periods of 
upheaval and depression—in the vast metamorphic changes effected 
throughout many districts—in the upward limita- 
tion of faults (fig. 62), as sometimes seen—and, 
briefly, in the worn and denuded surface which a 
lower formation often presents in connexion with 
Strata resting conformably upon it,—we have ad- 
ditional evidence of the lapse of long intervals of 
time during the elaboration of these rocks generally. 
But a still more conclusive proof of this fact is 
to be found in the limited vertical distribution of fossil species of 
plants and animals, the remains of which are entombed in so many of 
the sedimentary rocks. The sediments now under process of deposi- 
tion in our lakes, river-estuaries, and seas, frequently enclose, it will 
be remembered, the more durable parts, if not the entire forms, of 
various plants and animals belonging to existing creations. In like 
manner, the sedimentary deposits of former geological periods haye 
enclosed also various organic forms peculiar to those periods. Each 
group of strata has thus its own characteristic fossils, except that in 
the lowest or earliest-formed series of deposits we meet with no traces 
of ancient life. These deposits belong to the Azote Age of geological 
ows 
