26 
CAUSES OF 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 Arms 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 
