26 CAUSES OP THE SUPERPOSITION [Ch. Ill, 



the circumstances under which the secondary and tertiary 

 series originated, it is quite natural that particular tertiary 

 groups should occupy areas of comparatively small extent, 

 that they should frequently consist of littoral and lacustrine 

 deposits, and that they should often contain those admixtures 

 of terrestrial^ freshwater, and marine remains, which are so 

 rare in secondary rocks. It might also be expected, that the 

 tertiary volcanic formations should be much less exclusively 

 submarine, and this we accordingly find to be the case. 



CAUSES OF THE SUPERPOSITION OF SUCCESSIVE FORMATIONS 

 HAVING DISTINCT MINERAL AND ORGANIC CHARACTERS, 



But we have still to account for those remarkable breaks in 

 the series of superimposed formations, which are common both 

 to the secondary and tertiary rocks, but are more particularly 

 frequent in the latter. 



The elucidation of this curious point is the more important, 

 because geologists of a certain school appeal to phenomena of 

 this kind in support of their doctrine of great catastrophes, 

 out of the ordinary course of nature, and sudden revolutions 

 of the globe. 



It is only by carefully considering the combined action of all 

 the causes of change now in operation, whether in the animate 

 or inanimate world, that we can hope to explain such compli- 

 cated appearances as are exhibited in the general arrangement 

 of mineral masses. In attempting, therefore, to trace the 

 origin of these violations of continuity, we must re-consider 

 many of the topics treated of in our two former volumes, such 

 as the effects of the various agents of decay and reproduction, 

 the imbedding of organic remains, and the extinction of 

 species. 



Shifting of the Areas of Sedimentary Deposition. By re- 

 verting to our survey of the destroying and renovating agents, 

 it will be seen that the surface of the terraqueous globe may 

 be divided into two parts, one of which is undergoing repair, 

 while the other, constituting, at any one period, by far the 



