CHRONOLOGICAL SUCCESSION. 4I 
fossils, geologists have been able to divide the entire stratified 
series into a number of different divisions or formations, each 
characterised by a gezera/ uniformity of mineral composition, 
and by a special and peculiar assemblage of organic forms. 
Each of these primary groups is in turn divided into a series of 
smaller divisions, characterised and distinguished in the same 
way. It is not pretended for a moment that all these primary 
rock-groups can anywhere be seen surmounting one another 
regularly.* There is no region upon the earth where all 
the stratified formations can be seen together; and, even 
when most of them occur in the same country, they can 
nowhere be seen all succeeding each other in their regular and 
uninterrupted succession. The reason of this is obvious. 
There are many places—to take a single example—where one 
may see the the Silurian rocks, the Devonian, and the Carbon- 
iferous rocks succeeding one another regularly, and in their 
proper order. This is because the particular region where this 
occurs was always submerged beneath the sea while these for- 
mations were being deposited. There are, however, many 
more localities in which one would find the Carboniferous 
rocks resting unconformably upon the Silurians without the 
intervention of any strata which could be referred to the 
Devonian period. This might arise from one of two causes: 
1. The Silurians might have been elevated above the sea im- 
mediately after their deposition, so as to form dry land during 
the whole of the Devonian period, in which case, of course, 
no strata of the latter age could possibly be deposited in that 
area. 2. The Devonian might have been deposited upon the 
Silurian, and then the whole might have been elevated above 
the sea, and subjected to an amount of denudation sufficient to 
remove the Devonian strata entirely. In this case, when the 
land was again submerged, the Carboniferous rocks, or any 
younger formation, might be deposited directly upon Silurian 
strata. From one or other of these causes, then, or from subse- 
quent disturbances and denudations, it happens that we can 
* As we have every reason to believe that dry land and sea have existed, at 
any rate from the commencement of the Laurentian period to the present day, 
it is quite obvious that no one of the great formations can ever, under any cir- 
cumstances, have extended over the entire globe. In other words, no one of 
the formations can ever have had a greater geographical extent than that of 
the seas of the period in which the formation was deposited. Nor is there any 
reason for thinking that the proportion of dry land to ocean has ever been 
materially different to what it is at present, however greatly the areas of sea 
and land may have changed as regards their place. It follows from the above, 
that there is no sufficient basis for the view that the crust of the earth is com- 
posed of a succession of concentric layers, like the coats of an onion, each 
layer representing one formation. 
