SUCCESSION OF STRATA. 



185 



wide stratum of the sandstone formation, followed by a fo:mation 

 of marine limestone and shales and slate. It is by observations 



thus made near, or upon the surface, and carefully compared by 

 means of the fossils which are found imbeded in them, that ge- 

 ologists classify the various groups in the order we shall presently 

 name, and, as each stratum is characterized by its peculiar fossils, 

 and is never found taking the precedence of another which may 

 be anywhere found before it, they are justly supposed to be the 

 productions of different epochs. Thus, at one period of the earth's 

 history, peculiar limestones, sandstones, marls, and clays, were 

 deposited over the whole globe, upon those portions covered by 

 water; in which the zoophytes, fishes, drifted wood, plants, rep- 

 tiles &c., characteristic of that epoch were imbedded. At an- 

 other and previous period, were deposited the shales, the mill- 

 stone grit, and the immense beds of coal, imbedding peculiar and 

 distinct remains of animals and vegetables characteristic of that 

 epoch. At a still earlier period we find other distinct sedimentary 

 rocks, and under the whole, although sometimes appearing on the 



