196 GEOLOGY. 



a surface depression since this time. On the contrary, it was sub- 

 sequently filled with sedimentary beds, and the depression occupied 

 by the present lake is the result of subsequent excavation, and is of 

 relatively recent origin. The deep bowing of the series explains, in 

 part, the distribution of the exposed portions of the Keweenawan 

 system, which comes to the surface chiefly about the Lake Superior 

 syncline (Fig. 77). As originally laid down, the system fell far short 

 of covering all of the Animikean, and its distribution, compared with 

 that of the preceding systems, seems to indicate that the land areas 

 of the northern interior were more extensive in the Keweenawan period 

 than during either of the periods preceding. 



After the accumulation of the enormous thicknesses of Keweenawan 

 sediments, the region about Lake Superior suffered deformation, appar- 

 ently in common with other parts of the continent, and this event is 

 regarded as marking the end, or the beginning of the end, of the Pro- 

 terozoic 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 deposited 

 were brought into such an attitude as to be subject to erosion, but this 

 change did not cause great deformation of the strata. They are gener- 

 ally tilted, and locally, folded and faulted as well (Figs, 78 and 79), 

 though this is not the rule. In keeping with their structure, the rocks 

 are not metamorphic. The sandstones, graywackes, and conglomerates 

 are sandstones, graywackes, and conglomerates still. The igneous 

 rock of the series likewise remains, for the most part, in its massive 

 state, not having been made schistose after the fashion of the igneous 

 rocks intruded into the Huronian, and to a lesser extent into the 

 Animikean system. 



After the warping which followed the deposition of the Kewee- 

 nawan formation, the exposed surfaces of Keweenawan, Animikean, 

 Huronian, and Archean rocks suffered protracted erosion. Ultimately 

 the land about Lake Superior again sank, and when the sea came 

 back, a new series of sedimentary beds was deposited unconforma- 

 bly on the eroded surface of those which had preceded. When the 

 sea came back, its waters teemed with life, for the formation then 

 deposited contains abundant fossils. This abundantly fossiliferous 

 formation, resting unconformably on those which had preceded, is 

 classed with the Cambrian system, the oldest system of the Paleozoic 



