Dr. Mac Culloch on the Analogies of Rocks. 201 



we have here an analogy to the rocks of a coal series. The mud 

 with its plants, or shells, represents the different shales and 

 limestones ; and the sandstone is the counterpart of the sand 

 bed. The whole requires consolidation only, to render it an or- 

 dinary series of rocks. 



In sinking through the ancient estuaries of the sea, long filled 

 up and converted into dry land, similar beds of mud and clay, of 

 marine shells entangled in mud, and of sand and gravel, are 

 found ; varying in number, in thickness, in the order of repe- 

 tition, and in the quality or nature of the remains, in almost every 

 place. It is unnecessary to point out more distinctly a similar 

 analogy in these preparations for a series of secondary rocks ; but 

 it obviously requires only a repetition of the same deposits, suf- 

 ficiently frequent, to produce the whole series of secondary 

 strata. At what period the act of consolidation may have taken 

 place, we have no means of knowing ; yet, as far as our obser- 

 vations have hitherto reached, we have no reason to think that any 

 extensive operations of this nature are now going on, excepting 

 those formerly mentioned. The process may possibly be too slow 

 to fall within the sphere of our investigation. 



If the more ancient strata have been formed from similar ma- 

 terials, they should possess an analogy to the secondary; and, 

 admitting such differences as may be accounted for by the circum- 

 stances of difference in respect to consolidation to which they 

 have been exposed, we should, among them, find a series of 

 alternations analogous to the sandstone, shale, and limestone of 

 the latest series. Such an analogy can indeed be traced, but it 

 is imperfect. It will, in fact, be seen, that a great part of these 

 differences is explained by admitting the effects of heat on them ; 

 and it may fairly be presumed that there have also been differ- 

 ences in the rocks from whence the materials of these strata were 

 originally deposited. But there is still one difference remaining, 

 of great importance, and on which some light may at least be 

 thrown, if it cannot be fully explained. It consists in the very 

 great disproportion of limestone in the two series ; that rock being 



