138 GEOGNOSY AND GEOLOGY. 
20. Relation of Paleontology to Geology. 
It will have been understood from the preceding remarks that 
paleontology forms one of the most important branches of geology. It is 
almost indispensable to the accurate determination of strata groups, since 
the organic creations of certain periods are found to be much more 
constant, and more generally distributed than the contemporaneous mineral 
deposits. As we shall see subsequently, sandstone, lime, clay, and mar! 
substance, alternate with each other in the most varied manner; and it 
would be’ a matter of the utmost difficulty, nay, of almost absolute 
impossibility, to decide without the assistance of organic remains, and upon 
simple petrographical characters, upon the relative age or the identity of 
strata in different regions of the world. Modifications in the fauna and 
flora of a certain period of time may, indeed, occur; but this is only in 
single cases, and especially in the more recent deposits, where, as already 
remarked, it is impossible to mistake a difference in the external influences 
on organization. 
21. Object of Geology, 
The object of geology, in connexion with paleontology, is indeed 
important, and interesting as important. It seeks to develope the 
geography of the earth, as it existed at various periods of time, to point 
out what extent was possessed by the sea, what by the continents, 
what course was held by rivers and streams, and by what inhabitants 
peopled. Geology, however, is not merely an interesting subject of study 
and investigation, it is one of extreme importance to practical life. <A 
rational system of mining is impossible without it, and how necessary is it 
in an agricultural point of view! ‘To very many sciences and arts it is 
of most exceeding value. 
IV. SPECIAL OREOGRAPHY. 
Special Oreography treats of the relations and peculiarities of the solid ¢ «st 
of the earth, and of the order in which the different rock species are grou, ed. 
The classification into formations, groups, systems, &c., is based on prre 
experience alone, since the laws prevailing in regard to the composition of 
the earth’s exterior cannot be developed by hypothesis. This sectior of 
zeology is naturally subdivided into two parts, one having reference to 
normal rocks, the other to abnormal. 
NORMAL ROCKS. 
Normal rocks are especially characterized by possessing and exhibiting 
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