1877.] Critical Periods in the History of the Earth. 551 
The Quaternary, a Critical Period. We have given examples 
of several general unconformities, the signs of wide-spread oscil- 
lations of the earth-crust, attended with increase and decrease of | 
land, and therefore with great and wide-spread changes of cli- 
mate and, other physical conditions, and also with great and 
rapid changes of organic species. These times of general oscilla- 
tion are therefore the natural boundaries of the Eras or pri- 
mary divisions of Time. We have called them critical periods, 
transition periods, periods of revolution, because they are times 
of rapid change, both in the physical and the organic world, — 
a change overthrowing an old and establishing a new order of 
things. They are also times of lost record. We have seen that 
these critical periods, in comparison with the preceding and suc- 
ceeding, are continental periods, and it is for this reason that 
their record is usually lost. 
Now, the Quaternary is such a critical or transition period, 
marking the boundary between two great eras. The Quarter- 
nary is also a period of great and wide-spread oscillations, with 
increase and decrease of land,—a period of upheaval, erosion, 
down-sinking, to rise again slowly to the present condition. 
The early Quaternary was, therefore, to a marked degree a 
continental period. Here also we have newer rocks lying un- 
conformably on the eroded edges of an older series — river sedi- 
ments in old river-valleys, marine sediments in fiords; in other 
‘words, we have unconformity on a grand scale. Also, in connee- 
tion with these oscillations, we have great changes in physical 
geography, and corresponding and very wide-spread changes in 
climate, and consequently corresponding rapid changes in organic 
forms. Here, then, we have all the characteristics of one of the 
boundaries between the primary divisions of time. We have a 
transition or critical period, —a period corresponding to one of 
the lost intervals ; only in this instance, being so recent and being 
also less violent than the preceding ones, it is not lost. From 
this it follows that the study of the Quaternary ought to furnish 
the key which will unlock many of the mysteries which now 
trouble us. Some of the problems which have been or will be 
Daaa by study of the Quaternary we will now briefly men- 
on. 
I. Changes of Species not sudden. If the Quaternary were 
lost, and we compare the Tertiary rocks with the unconforma- 
bly overlying recent rocks, and the Tertiary mammals with those 
now living, how great and apparently sudden seems the change ! 
