THE PROTEROZOIC ERA 465 



Dwelling on these aspects of the case, we almost reach the 

 conclusion that the thickness of a series of beds, as we usually 

 measure thicknesses where they are great, is independent of the depth 

 of the basin. This, however, is pushing the case too far, for the 

 considerations that have been urged apply only to deposition of 

 the kinds specified, i. e., deposition on appreciable slopes. As origin- 

 ally laid down, the Keweenawan system probably fell far short of 

 covering all of the Animikean. 



Deformative movements. The sedimentary part of the Kewee- 

 nawan series has usually been thought to imply marine submergence; 

 but so far as now known the sediments may have been accumulated 

 in an interior basin and may be more or less subaerial. It was 

 probably during and at the close of this period that the Lake Superior 



Fig. 356. Diagram illustrating the devlopment of the Lake Superior syn- 

 cline. ^52, Archean; H and A, Huronian and Animikean; K, Keweena- 

 wan. (Irving, U. S. Geol. Surv.) 



syncline (Fig. 356) was formed, in part at least, and the Kewee- 

 nawan rocks tilted toward its axis, both from the north and south. 

 It is not to be understood that this syncline has remained a surface 

 depression ever since. On the contrary, it was subsequently filled 

 with sedimentary beds, and the basin occupied by the present lake 

 is the result of later and relatively recent excavation. The deep 

 bowing of the series explains, in part, the distribution of the ex- 

 posed portions of the Keweenawan system, which comes to the 

 surface chiefly about the Lake Superior syncline. 



The deformation of the Keweenawan strata of the region about 

 Lake Superior was perhaps contemporaneous with deformation in 

 other parts of the continent, and these changes are regarded as 

 marking the beginning of the end of the Proterozoic era in 

 this region. 



As a result of the deformation at the close of the Keweenawan, 

 some parts of the area where Keweenawan sediments had been 



