1877.] Critical Periods in the History of the Earth 548 
spread oscillation is also apparent, for all the places mentioned 
were sea-bed in Archean, land during the interval, and again 
sea-bed during the Silurian. But of this long interval not a leaf 
of record remains. 
Evidently, then, at the end of the Archean an enormous area 
of Archean sea bottom was raised up and crumpled, and became 
land. After remaining land for a time sufficiently long to allow 
enormous erosion of crumpled strata it again went down to the 
old Primordial shore line, and the Silurian age commenced. This 
time of elevation is the lost interval. 
Now, when the record closed in the Archean, as far as we 
know, only the lowest forms of Protozoan life yet existed. The 
beginnings of life had not yet differentiated into what might be 
called a fauna and flora. When the record again opened with 
the Primordial we had already a varied and highly organized 
fauna, consisting of representatives of many classes and of all the 
great types of animal structure except vertebrates, Nor were 
these representatives the lowest in three several departments, for 
Trilobites and Orthoceratites can hardly be regarded as lower 
than the middle of the animal scale as it now exists. It is cer- 
tain, therefore, that all the great departments except vertebrates, 
and most of the classes of these departments, including animals at 
least half-way up the animal scale, were differentiated during the 
lost interval. The amount of evolution during this interval can- 
not be estimated as less than all that has subsequently taken 
place. Measured by the amount of evolution, this lost interval 
is equal to all the history of the earth which has since elapsed. 
We escape this very improbable conclusion only by admitting 
4 more rapid rate of evolution during critical periods. 
It is one of the chief glories of American geology to have first 
established the Archzan as one of the primary divisions of time. 
t is even yet reluctantly admitted as such by many European 
geologists. And yet it is seen that from every point of view, 
Whether of the rock system or of the life system, it is by far the 
most widely and trenchantly separated of all the eras. 
The next greatest lost interval (though far less than the pre- 
ceding) is that between the Palwozoic and the Mesozoic. Here 
We have the next most general unconformity, indicating the next 
most wide-spread changes of physical geography and climate, ac- 
companied by the most sweeping changes in organic forms, not 
only in Species and genera, but also in families and orders. This 
change is the more striking as it occurs in the midst of an abun- 
