322 C. W. HALL — KEWEENAWAN AREA OF EASTERN MINNESOTA 



The unconformity between this series and the overlying Cambrian is 

 most beautifully and typically shown at two or three places. This has 

 become a classic spot for the downward delimitation of the Cambrian in 

 the Mississippi valley. 



The clastic rocks of the Keweenawan at Taylors falls are unique. 

 They consist of a succession of volcanic breccias and tufis alternating 

 with the compact ophitic diabase of the lava flows. Tlie breccias con- 

 sist of angular fragments of diabase "imbedded in a matrix of finely 

 crystalline secondary minerals, chiefly epidote and quartz."-'^ Tbey 

 occur quite statedly between the successive flows, and when once recog- 

 nized are an excellent guide for the recognition of the division planes 

 between them. The tuffs do not occur so constantly. They are not in 

 extensive masses ; one jilace shows a thickness of breccia and tuff of 25 

 feet to 30 feet; elsewhere the thickness is materially less. The "par- 

 ticles vary in size from mere dust to the size of an ordinary sand grain, 

 and in the amount of a])ra8ion to which they have been subjected from 

 roughly angular to beautifully rounded grains." f 



Doctor l^erkey observes that this is one of the localities, noted in the 

 geological literature of the Lake Superior district, where a well defined 

 tuff derived from volcanic ash occurs. 



No interbedded conglomerates have been detected at Taylors falls. 



THE SAINT CROIX RIVER SECTION 



This section comprises the rocks around the junction of the Snake and 

 Kettle rivers with the Saint Croix. The farthest exposure upstream at 

 the Kettle River rapids, and consequently the lowest rock seen in this dis- 

 trict, is a conglomerate. Its thickness seems considerable. The texture 

 is that usual in the series. At the time the locality was visited high water 

 l)revented a careful examination, At this })oint, and continuously down 

 the river for some miles, the best exposures are on the Wisconsin side and 

 the numerous islands along this portion of the river. The conglomerate 

 named is followed by one or more lava flows ; after these another con- 

 glomerate bed occurs; other lava flows, follow this, and then another 

 conglomerate bed. The entire succession in this group of exi)()sures 

 strikes steadily south 10 to 15 degrees west and dips westerly at an 

 average angle of 12 degrees. This last named exposure is in section 29, 

 township 38, range 19 west, and has ])een explored quite extensively 

 for copper, as have the associated diabases, by Mr D. A. Caneday, who 

 has given much valuable information on the geology of the district. 

 •The thickness of the rocks along the Saint Croix at this point cannot be 



*Ibid,, vol. xxi, 1898, p. 14f). 

 t Ibid., p. 140. 



