• , ERUPTIVES AND FAULT LINE 337 



period was repeatedly interrupted, but not suspended, by the igneous 

 outpourings.^ It is more in harmon}^ with the accepted basis of geolog- 

 ical chronometry to regard the period as one of long-continued detrital 

 deposition interjected with the many separate beds of outpoured lava. 

 The cataclysms incident to the formation of these lava flows, though 

 striking in their results, were very brief in their activity and local in 

 their prevalence, while the detrital accumulations must have been vast 

 in their periods and widely represented by equivalent deposits. 



Development of the Fault Line 



Interpreting events in the light of the foregoing discussion, the follow- 

 ing sequence is suggested in the development of the great fault line : 



A line of weakness stretches westward and southward from the Lake 

 Superior basin. West of the southern 50 or more miles there lay a belt 

 of Keewatin sediments and eruptives not less than 15,000 feet in thick- 

 ness extending into central Minnesota, while to the east of it there is not 

 a trace of this load. Such a line of weakness once clearly defined by 

 crustal conditions would naturally determine the position of that long 

 line of volcanic vents necessary to afford the supply of lava in the flows 

 of Minnesota and Wisconsin, so remarkable in number and long stretch 

 of surface covered by them. A fault line was clearly determined, either 

 before the lava was poured out or during the long period of alternating 

 volcanic activity and sea erosion represented by the rocks herein de- 

 scribed. The outpouring of lavas conspired with the uplift of the sur- 

 face southeast of the fault line to raise the contiguous edge of the flows 

 and tilt the rocks into the position they have assumed. The Cambrian 

 sandstones, accumulating probably from areas eroded to the north and 

 west, filled the sunken areas and overwhelmed the lava flows. The 

 sliding of the rocks continued, and these sandstones became involved 

 in the movement, so that now the junction of the Cambrian and Kewee- 

 nawan is a fault-line contact along which the latter is crushed and shat- 

 tered for 400 feet and more, according to Professor Grant,t and the 

 former is profoundly displaced J or broken and dislocated into blocks 

 in such hopeless confusion that the formation attitude can not be deter- 

 mined. The amount of displacement is great ; at Ashland, at least 3,500 

 feet ; at Chengwatana, how much more than 700 feet can not yet be told. 

 That the faulting is great is further indicated by the heavy dip south- 



* Geology of Wisconsin, vol. iii, 1880, p. 393. 



t Preliminary report on the copper-bearing rocks of Douglas county, Wisconsin, p. 18 and pi. v. 

 Published by the State, 1900. 

 X Ibid., p. 20 and pi. vi. 



XLVIII— Eur.T,. Geoi-. Soc. Am., Vor.. 12, 1900 



