152 : GEOLOGY. 
already have of the immense periods presented to our contemplation by 
geology. 
The Relative Ages of Rocks—When we speak of the ages of rocks, 
we can do so only by comparison with others. Our ideas on this point 
are merely relative. We can assert, as we have already done in analysing 
the rock-sections, that one layer must have been formed before another ; 
or that, after its formation, and before the deposition of a certain other 
rock, a rise or fall in the strata took place; or that, at a certain point 
in the series, a volcanic eruption threw up a mass of igneous rock; and 
make like statements based on comparison of the rocks with one another. 
Our conceptions, therefore, regarding the connection in age between the 
various rock-formations are merely relative, one being proved to have 
taken place before, or after, or during the formation of another. 
The Order of the Rock-formations——By long-continued and widely 
extended observations in various parts of the globe, based on numberless 
data of composition, structure, inclination, and fossil contents, geologists 
have been able to form a definite list of the various rock-formations from 
the earliest to the most recent, arranged in the order of time. They have 
divided the whole of the rocks composing the crust of the earth into 
sections called ‘systems, and these again into ‘ groups,’ in a certain well- 
defined order—so that when a rock is presented to their observation in 
any part of the globe, they can state, with more or less certainty, the 
system to which it belongs, and the period in the past history of the 
earth at which it was deposited. Regarding these rock-systems, one 
point is to be very strictly noted. Suppose that we ,represent the 
various rock-systems by the letters of the alphabet—the earliest by A, 
the second by B, and so onwards to the last and most recent, represented 
by Z. Now, the various rock-systems always stand in this relative his- 
toric order; so that the formation indicated by M is always after L, 
and before N, and cannot occur in any other relation to these two 
systems, wherever they may be found. At the same time, certain for- 
mations, one or more, may be awanting in certain parts of the world, 
not having been deposited there; so that one or more systems may 
not be represented in these districts. Thus, L and M may be awanting. 
What two systems will then be found together? Certainly and unvary- 
ingly, K and N. But here the historic order is not violated, as it would 
be if N preceded K. The various rock-systems are, therefore, always 
“presented in an unvarying succession in the order of their formation ; 
although in different parts of the globe certain strata, and even whole 
systems, may not be found. 

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