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LYELL'S ELEMENTS OF GEOLOGY. 



Tests of the different Ages of Aqueous Rocks. 



these exceptional cases. When he finds that the strata are frac- 

 tured, curved, inclined, or vertical, he knows that the original 

 order of superposition must be doubtful, and he will endeavour 

 to find sections in some neighbouring district where the strata 

 are horizontal, or only slightly inclined. Here it is impossible 

 that they can have been extensively thrown over and turned 

 upside down, for such a derangement cannot have taken place 

 throughout a wide area without leaving manifest signs of dis- 

 placement and dislocation. 



Mineral character. — The same rocks may often be observed 

 to retain for miles, or even hundreds of miles, the same mineral 

 peculiarities, if we follow them in the direction of the planes of 

 stratification. But this uniformity ceases almost immediately, if 

 we pursue them in an opposite direction. In that case we can 

 scarcely ever penetrate a stratified mass for a few hundred yards, 

 much less several miles, without beholding a succession of ex- 

 tremely dissimilar calcareous, argillaceous, and siliceous rocks. 

 These phenomena lead to the conclusion, that rivers and currents 

 have dispersed the same sediment over wide areas at one period, 

 but at successive periods have been charged, in the same region, 

 with very different kinds of matter. The first observers were 

 so astonished at the vast spaces over which they were able to 

 follow the same homogeneous rocks in a horizontal direction, 

 that they came hastily to the opinion, that the whole globe had 

 been environed by a succession of distinct aqueous formations, 

 disposed round the nucleus of the planet, like the concentric 

 coats of an onion. But although, in fact, some formations may 

 be continuous over districts as large as half of Europe, or even 

 more, yet most of them either terminate wholly within narrower 

 limits, or soon change their lithological character. Sometimes 

 they thin out gradually, as if the supply of sediment had failed 

 in that direction, or they come abruptly to an end, as if we had 

 arrived at the borders of the ancient sea or lake which served 

 as their receptacle. It no less frequently happens that they vary 

 in mineral aspect and composition, as we pursue them horizon- 

 tally. For example, we trace a limestone for a hundred miles, 

 until it becomes more arenaceous, and finally passes into sand, 

 or sandstone. We may then follow this sandstone, already proved 

 by its continuity to be of the same age, throughout another dis- 

 trict a hundred miles or more in length. 



Organic remains. — This character must be used as a crite- 

 rion of the age of a formation, or of the contemporaneous origin 

 of two deposits in distant places, under very much the same 

 restrictions as the test of mineral composition. 



First, the same fossils may be traced over wide regions, if 



