102 J. LeConte—Critical Periods in the History of the Earth. 
erosion. That it was a period of wide-spread oscillation is also © 
evident ; 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 Archzean an enormous area 
of Archzean sea-bottom was raised up and crumpled and became 
land. After remaining land for a time sufficiently long to_ 
allow enormous erosion of the crumpled strata it again went 
down to the old Primordial shore-line and the Silurian age com- 
menced. 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. en the record again 
opened with the Primordial we have already a varied an 
highly organized fauna, consisting of representatives of many 
classes and of all the great types of animal structure, except 
vertebrates. Nor are these representative 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 certain therefore that all the great 
departments except vertebrates, and most of the classes of these 
departments including animals as least half way up the animal 
scale, were differentiated during the lost interval. The amount 
of evolution during this interval cannot be estimated as less 
than all that has taken place since. Measured by the amount 
of evolution therefore, this lost interval is equal to all the his- 
tory of the earth which has elapsed since. We escape this very 
improbable conclusion, only by admitting a more rapid rate of 
evolution during critical pervods. 
It is one of the chief glories of American Geology, to have 
first established the Archean as one of the primary divisions of 
time. It is even yet re admitted as such by many 
we have the next most general unconformity, indicating the 
next most wide-spread changes of physical geography and cli- 
