CHRONOLOGICAL SUCCESSION. 41 



fossils, geologists have been able to divide the entire stratified 

 series into a number of different divisions or formations, each 

 characterised by a general 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 : 

 I. 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. 



