EVIDENCES OF TILTING 673 



beds as the flow series was accumulated. The Huron ian iron formation, 

 which dipped south 20 to 30 degrees at the time of the first flow, was, if 

 the above interpretation is correct, gradually tilted back so that it had a 

 horizontal position when the flow series had attained a thickness of about 

 2 miles, for flows that far up in the series are parallel to the iron forma- 

 tion. The tilting continued so that when the uppermost flow was ex- 

 truded on the south-sloping surface the lowermost flow dipped about 50 

 degrees to the north. The tilting continued during the deposition of the 

 Upper Keweenawan sandstones. 



Other Facts to be fitted into the Stoey 



Numerous dikes cut the Huronian and some of the lower Keweenawan 

 flows. They evidently flUed major joints in the main, for they have a 

 general parallelism. In general, they pitch downward to the east about 

 15 degrees from the horizontal and dip about 45 degrees to the south. 

 Thus their strike at the surface is now northeast where the iron forma- 

 tion strikes east. 



In the lower part of the iron formation is a great bed fault which dis- 

 places the dikes and the younger part of the formation upward and east- 

 ward 800 to 900 feet in the westernmost mines and 500 feet in the region 

 of Bessemer. 



A great gabbro laccolith extends from the Wisconsin-Michigan line 

 westward for 50 miles. It maintains a thickness of 2 miles in its western 

 part, and west of Mineral Lake bulges to a thickness of 4 miles. Field 

 work in 1923 indicates that this is not a single laccolite, but rather a 

 series of parallel laccolitic intrusions into the lower part of the flow 

 series. 



Another fact to fit into the story is the great abundance of acid sedi- 

 ments interspersed between the flows. According to Lane, in the Besse- 

 mer section the amount of these is much greater than the total mass of 

 acid igneous rocks now visible, and also much greater in amount than 

 the basic sediments. Acid conglomerates exist in Wisconsin directly 

 associated with acid flows, and it is believed that the sediments represent 

 partial erosion of such flows, which, however, did not get so far from the 

 volcanic source as the sediments. 



Hypothesis explaining the foregoing Facts 



GENERAL iSTATEMENl' 



The origin and structure of the rocks of the Lake Superior syncline fit 

 in with the story of a great bathylith. It is my conception that this 



