1877. ] Critical Periods in the History of the Earth. 541 
Evidently, therefore, every case of unconformity marks a period 
of time — often a long period —during which there was no 
record made in strata and fossils at the observed place; certain 
leaves — frequently very many —are there missing from the Book 
of Time. Is it any wonder, then, that skipping over these pages 
when we commence reading again we find the matter entirely 
new? Evidently, the suddenness of the change in organic forms 
is only apparent. If we could recover the record, which was 
doubtless carried on elsewhere, the break would disappear; if 
we could find the missing leaves the reading would be continu- 
ous. In every such instance, therefore, there is a lost interval of 
history. In cases of local unconformity we recover the lost rec- 
ord in other places, and thus fill up the blank in the history. 
But in some cases of very general unconformity, such as those 
which mark the great divisions of time, the loss is not yet recov- 
ered, perhaps is irrecoverable, though doubtless the more com- 
plete knowledge of the geology of the whole earth surface will go 
far toward filling blanks and making the record continuous. 
The view above presented is now held by all geologists, but 
there seems to be danger, under the influence of the now domi- 
nant views of evolution, of erring on the other extreme. Assum- 
ing a uniform rate of evolution, many, it seems to me, commit 
the mistake of measuring the amount of lost interval by the 
amount of change of organic forms, and thus discredit the real 
value of the geological record by exaggerating greatly its frag- 
mentary character. On the contrary, there appears good reason 
to believe that the evolution of the organic kingdom, like the 
evolution of society and even of the individual, has its periods of 
rapid movement and its intervals of comparative repose and read- 
justment of equilibrium. Geological history, like all other his- 
tory, has its periods of comparative quiet, during which the forces 
of change are gathering strength, and periods of revolution, dur- 
ing which the accumulated forces manifest themselves in conspic- 
uous changes in physical geography and climate, and therefore 
m rapid movement in the mareh of evolution of organic forms, — 
periods when the forces of change are potential, and periods 
When they become active. Conformable rocks represent the 
intervals of comparative quiet, during which organic forms are 
either permanent or change slowly; unconformity represents a 
_ Aime of oscillation, with increase and decrease of land, and there- 
fore of rapid changes of physical conditions and correspondingly 
rapid movement in evolution. The general unconformities, of 
