200 A. W. G. WILSON TRAP SHEETS OF LAKE NIP1GON BASIN 



earlier rocks, varies from medium to coarse; usually it is nearly uniform 

 throughout the thickness of the whole sheet, except close or near to the 

 basal and upper contacts, even where the sheets are several hundred feet 

 in thickness. The sheets of diabase, with a few local exceptions, are re- 

 markably constant in their petrographic characters over nearly the whole 

 area. They are invariably noncrystalline and are never amygdaloidal. 



VIEWS AS TO THEIR ORIGIN 



The earlier students of the district regarded these diabase sheets, in 

 whole or in part, as volcanic flows, the lower and thinner intrusive sheets 

 being considered as contemporaneous with the sedimentary rocks with 

 which they are associated, while the capping sheet was designated the 

 "Crowning overflow" by Sir William Logan. Later work by Ingall 2 and 

 by Lawson 3 has combatted these ideas, and the view so admirably set 

 forth by Lawson has come to be generally accepted. 



Lawson's thesis is (page 29) : 



"There are no contemporaneous volcanic rocks in the Animikie group. 



"None of the trap sheets associated with the Animikie, whether of the na- 

 ture of 'caps' or intercalated sheets, is a volcanic flow. 



"These trap sheets are all intrusive in their origin and are of the nature of 

 laccolitic sills." 



Lawson further summarizes the data on which he founds this thesis 

 in the following statement (page 44) : 



"I. The trap sheets associated with the Animikie strata are not volcanic 

 flows because of the combination of the following facts : 



"1. They are simple geologic units, not a series of overlapping sheets. 



"2. They are flat, with uniform thickness over more than one hundred 

 square miles in extent, and where inclined the dip is due essentially to fault- 

 ing and tilting. 



"3. There are no pyroclastic rocks associated with them. 



"4. They are never glassy. 



"5. They are never amygdaloidal. 



"6. They exhibit no flow structure. 



"7. They have no ropy or wrinkled surface. 



"8. They have no lava breccia associated with them. 



"9. They came in contact with the slates after the latter were hard and 

 brittle and had acquired their cleavage, yet they never repose upon a surface 

 which has been exposed to subaerial weathering. 



"II. They are intrusive sills because of the combination of the following 

 facts : 



2 E. D. Ingall : "Mines and mining on lake Superior." Geological survey of Canada, 

 1888, vol. ii, part 2, section H, pp. 42, 46,. 79, 80, 99. 



3 A. C. Lawson : "The Laccolitic sills of the northwest coast of lake Superior." Minne- 

 sota Geological Survey, Bulletin viii, 1893. 



