Ch. IX.] OF AQUEOUS EOCKS. 93 



their position alone, the order in which the events recorded in them have 

 occurred. 



In regard to the crust of the earth, however, there are some regions 

 where, as the student has already been informed, the beds have been dis- 

 turbed, and sometimes extensively thrown over and turned upside down. 

 (See pp. 58, 59.) But an experienced geologist can rarely be deceived 

 by these exceptional cases. "When he finds that the strata are fractured, 

 curved, inclined, or vertical, he knows that the original order of superpo- 

 sition must be doubtful, and he then endeavors to find sections in some 

 neighboring district where the strata are horizontal, or only slightly in- 

 clined. Here the true order of sequence of the entire series of deposits 

 being ascertained, a key is furnished for settling the chronology of those 

 strata where the displacement is extreme. 



Mineral character. — The same rocks may often be observed to retain for 

 miles, or even hundreds of miles, the same mineral peculiarities, if we fob 

 low the planes of stratification, or trace the beds, if they be undisturbed, in 

 a horizontal direction. But if we pursue them vertically, or in any direc- 

 tion transverse to the planes of stratification, this uniformity ceases almost 

 immediately. In that case we can scarcely ever penetrate a stratified mass 

 for a few hundred yards without beholding a succession of extremely dis- 

 similar rocks, some of fine, others of coarse grain, some of mechanical, others 

 of chemical origin; some calcareous, others argillaceous, and others silice- 

 ous. 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 differ- 

 ent 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 forma- 

 tions, 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 sedi- 

 ment 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 horizontally. For example, 

 we trace a limestone for a hundred miles, until it becomes more arena- 

 ceous, and finally passes into sand, or sandstone. We may then follow this 

 sandstone, already proved by its continuity to be of the same age, through 

 out another district a hundred miles or more in length. 



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

 eral composition. 



First, the same fossils may be traced over Avide regions, if we examine 



