THE PROTEROZOIC ERA. 181 



(p. 150). The structure of these formations is so complex, as a result of fault- 

 ing and folding, that thicknesses are not easily estimated. The thickness of the 

 Knife Lake formation has been estimated at 2500 feet, which is probably less 

 than its original thickness. 



In the area north of Lake Huron, whence the Huronian system derives its 

 name, the strata formerly included in it consist of great thicknesses of rock, divisi- 

 ble into thirteen formations. 1 Though outside the Lake Superior region, the 

 section is here given. In ascending order the formations are as follows: 



(1) Gray quartzite, 500 feet; (2) chlorite slate (altered surface volcanic rock), 

 2000 feet; (3) white quartzite, 1000 feet; (4) lower slate conglomerate (or gray- 

 wacke and graywacke conglomerate), 1280 feet; (5) limestone, 300 feet; (6) 

 upper slate conglomerate (or graywacke and graywacke conglomerate), 3000 

 feet; (7) red quartzite, 2300 feet; (8) red jasper conglomerate, 2150 feet; (9) 

 white quartzite, 2970 feet; (10) yellow cherty limestone, 200 feet; (13) white 

 quartzite, 400 feet; total, 18,000 feet. Some of this is now classed as Animikean. 



The first five of the above formations, with an aggregate thickness of about 

 5000 feet, were formerly regarded as " Lower Huronian " 2 and the others as " Upper 

 Huronian." It is now considered possible that all may be Huronian, as that 

 term is here used. 3 The basal member of this series rests on the Archean uncon- 

 formably, and there appears to be an unconformity between the fifth and sixth 

 members. The uppermost member is overlain unconformably by Cambrian beds. 



It is to be borne in mind that the series, of rocks here included under the Huro- 

 nian have but recently been divided into two systems, and that, if future work 

 shall show that the break between them is important and general, rather than 

 local, one of the divisions will doubtless receive an appropriate name and take its 

 place in the series along with Huronian, Animikean, and Keweenawan. 



Erosion of the deformed Huronian system. — The deformation of 

 the Huronian formations at the close of the period brought considerable 

 areas of what had been the sites of sedimentation into positions favor- 

 able for erosion. The extent of these areas is unknown, and is likely 

 to remain so, since most of the Huronian strata are now covered by 

 later beds which effectively conceal much of the eroded surface of 

 the Huronian, and prevent the determination of its extent. The 

 period of erosion which followed seems to have been long, but its dura- 

 tion cannot be accurately measured. It is certain that great thicknesses 

 of the sedimentary beds were carried away, for the stubs of great folds, 



1 Logan, Rept. of Progress of the Geol. Surv. of Canada, 1863. Van Hise, 16th 

 Ann. Rept., Pt. I, U. S. Geol. Surv., p. 776. 



2 Van Hise, Bull. 86, U. S. Geol. Surv. See also Jour, of Geol., Vol. XIII, p. 98. 



3 Coleman, American Geologist, Vol. XXIX, 1902, pp. 327-334. Willmott, Am. 

 Geol., Vol. XXVIII, pp. 14-19, thinks most of this Huronian equivalent to the "Upper" 

 Huronian of Canadian geologists. 



