542 Critical Periods in the History of the Earth. [September, 
course, mark times of very general commotion, — of wide-spread 
changes of physical geography and climate, and consequently of 
exceptionally rapid and profound changes in organic forms. 
These periods of revolution in all history are critical, and. 
hence are of especial interest to the philosophic historian and 
to the evolutionist ; but they are also in all history periods of 
lost record. And asin 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 examples 
of such lost intervals, and show their significance in evolution. 
The first and by far the greatest of these is that which occurs 
between the Archean and the Palxozoic. In every part of: the 
earth where the contact has yet been 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, the Hebrides, Bavaria, Bohemia, Scandinavia. Uncon- 
formity in such widely separated localities indicates wide-spread 
changes in physical geography, and therefore, presumably, of all 
those physical conditions included in the word climate. These 
changes of physical geography are best illustrated in the United 
States. The break between the Archæan and the Primordial 
has been observed in very many places all over the wide area of 
the United States, east and west: not only in Canada, in New 
York, in the Appalachian region, in Wisconsin, Missouri, At 
kansas, and Texas, but also all over the Rocky Mountain region, 
in Nebraska, Montana, Idaho, Wyoming, Colorado, Utah, Ne- 
vada, New Mexico, and Arizona. As upturned, eroded, outerop- 
ping strata mean land surface, it is evident that there was at 
that time a very large area or else several large areas of land in 
` the place now occupied by the American continent. In compat 
ison with the subsequent Silurian it was æ continental period. 
This land is often spoken of as Archæan land. It was in 
land of Archean rocks, but for that very reason not of Archean 
times, for these rocks were, of course, formed at the bottom of the 
sea in Archean times, and therefore these localities were all sea- 
bed receiving sediment at that time. We know absolutely noth- 
ing of the land of Archæan times, and never can know anything 
until we find still older rocks, from the débris of which Archæan 
sediment was formed. ‘The land spoken of above was land 2 
the Lost Interval. That the interval was immensely long is 6” 
dent from the prodigious erosion. That it was a period o 
