THE INTERVAL. 



295 



Archaean marginal sea-bottom was raised into land, and at the same 

 time crumpled. In c, the same was eroded, so that the edges of strata 



Arch yean Land 



Fig. 267.— Ideal Section, showing how Unconformity was produced on the Canadian Border: S L, 



sea-level. 



were exposed. This was during the interval. In d, the land sank 

 again to the primordial shore-line, and the Palaeozoic era commenced. 

 During the Palaeozoic the land gradually rose so as to expose successive 

 sea-bottoms of Cambrian, Silurian, Devonian, and Carboniferous ages. 

 It has remained substantially in this condition ever since. 



We have spoken thus far only of the uncomformity of the New 

 York rocks on the Canadian rocks. This phenomenon may be ex- 

 plained, as we have seen, by local oscillations, with increase and decrease 

 of land-area during the lost interval. But, when we remember that the 

 same unconformity is found in the most widely-separated localities, 

 over the whole area of the United States, we are forced to the conclu- 

 sion that the lost interval, as compared with the Silurian, was probably 

 a continental period — a period of widely-extended land composed of 

 Laurentian rocks. The whole of this land disappeared by submergence 

 at the beginning of the Primordial, except the Canadian area, a large 

 area east of the Appalachian, and probably a considerable area in the 

 Basin region, and perhaps a few islands or larger areas in the Primordial 

 seas between, as shown in map, Fig. 266. 



In all speculations on the origin of the animal kingdom by evolu- 

 tion, it is very necessary to bear in mind this lost interval, for it was 

 evidently of great duration. 



