202 



MINERALS AND GEOLOGY 



FIG. no. 



beyond the area of deposition to which the sediments extended. (See 

 some of the preceding observations under " Formation of Sedimen- 

 tary Rocks," " Denudation," &c.) 



(3) A formation of a given age may be represented in one place 

 by a limestone j in another, by a sandstone ; in a third, by argillace- 

 ous shales, and so on. This will be easily understood, if we reflect 

 that at the present day these different kinds of rock are being formed 

 simultaneously at different places. Many of our preceding observa- 

 tions have amply illustrated this, but the fact may be rendered still 

 clearer by the accompanying diagram. In this sketch, the dark out- 

 line is intended to 

 represent a s o m e- 

 what extended line 

 of coast, with a river 

 debouching into a 

 deep bay. In the 

 latter, the argilla. 

 ceous or muddy sedi- 

 ments (a), brought down by the river, may be deposited. At G, we 

 may suppose a granitic headland. The arenaceous or siliceous sedi- 

 ments (s) derived from the disintegration of this, will be arranged 

 along the shore beyond it, by the set of the current. Finally, at L, 

 we may suppose the occurrence of exposed cliffs of limestone, yielding 

 calcareous sediments (c). These various sedimentary matters will be 

 also in places more or less intermingled, producing rocks of inter- 

 mediate or mixed composition. But these rocks will be shown to be 

 of the same period of formation, by the identity of some, at least, of 

 the organic bodies contained in them : although many of the enclosed 

 shells, &c., will necessarily be distinct, owing to the diverse nature 

 of the sediments, the more or less exposed character of the coast, and 

 the varying depths of water prevailing at different places. We 

 might expect, moreover, to find in one and all of these deposits, 

 coins, pieces of pottery, and other objects of human workmanship, 

 proving both their contemporaneous and their recent origin. Hence, 

 the age of a rock, it must be remembered, is in no way indicated by 

 mineral composition : sandstones, limestones, &c., are of all geological 

 iperiods. 



(4) From time to time, during the gradual deposition of these 

 sedimentary formations, various eruptive rocks were driven up 



