480 CRITICAL PERIODS IN TEE HISTORY OF THE EARTH 



the Hebrides, Bavaria, Bohemia, Scandinavia. Unconformity in such wide- 

 ly separated localities indicates wide-spread changes inphj^sical 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 Archaean 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, Arkansas and Texas, but also 

 all over the Eockj" Mountain region, in Nebraska, Montana, Idaho, Wyo- 

 ming, Colorado, Utah, Nevada, New Mexico and Arizona. As upturned, 

 eroded, outcropping 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 comparison with the subse- 

 quent Silurian it was a continental period. This land is often spoken of as 

 Arclioian land. It was indeed land of Arc Java n rocks, but for that very reason 

 not of Archa3an tini(?s, for these rocks were, of course, formed at the bottom 

 of the sea in Archaean times, and therefore these localities were all sea-bed 

 receiving sediment at that time. We know absolutely nothing of the land 

 of Archfean times, and never can know anything until we find still older 

 rocks, from the debris oi which Archaaan sediment was formed. The land 

 spoken of above was land of the Lost Interval. That the interval was im- 

 mensely long is evident from the prodigious erosion. That it was a period 

 of wide-spread oscillation is also apparent, for all the places mentioned 

 were sea-bed in Archaean, 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 Archa3an an enormous area of Archaean 

 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 Archsean, 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 

 hio-hly 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 certain, therefore, that all the great de- 

 partments except vertebrates, and most of the classes of these departments, 

 including animals at least half-way up the animal scale, were differentiated 

 durino- the lost interval. The amount of evolution during this interval 

 cannot 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. AVe escape this very imjiroba- 



