J. LeConte—Critical Periods in the History of the Earth. 101 
formity represents a time of oscillation with increase and decrease 
of land, and therefore of rapid changes of physical conditions 
and correspondingly rapid movement in evolution. The general 
unconformities, of course, mark times of very general commo- 
tion—of wide-spread changes of physical geography and cli- 
mate, and therefore of exceptionally rapid and profound 
_changes in organic forms. 
These periods of revolution in all history are critical; and there- 
fore are of especial interest to the philosophic historian and to 
the evolutionist; but they are also in all history periods of Jost 
record. And as in human, so also in geological history, the 
farther back we go, the longer are the lost intervals and the 
more irrecoverable the lost records. We will now give exam- 
of such lost intervals and show their significance in evolu- 
on. 
The first and by far the greatest of these lost intervals is that 
which occurs between the Archean and the Paleozoic. In 
every part of the earth where the contact has been yet observed 
the Primordial lies unconformably on the upturned and eroded 
edges of the Archean strata. This relation was observed first 
in Canada, then in various parts of the eastern United States, 
then in Scotland, Hebrides, Bavaria, Bohemia, Scandinavia. 
Unconformity in such widely separated localities, indicates 
wide-spread changes in physical geography, and therefore pre- 
sumably of all those physical conditions included in the wor 
imate, These changes of physical ge hy are best illus- 
h 
an formed at the bottom of the sea in Archean times, and 
erefore these localities were all sea-bed receiving sediment at 
ti ttime. We know absolutely nothing of the land of Archean 
mes, and never can know anything until we find still older 
re from the debris of which Arch 
— 2, was land of t Interval. That 
the ot itm of above, was land of the Lost Interva 
