THE KEWEENAW AN. 409 



sediments. Ing'alls held that some of these masses were of intrusive origin. 

 Irving considered them all as intrusive. 



Chauvenet, in his manuscript notes, refers to a dike which he places 

 with the Logan sills, cutting the Animikie slates on Pigeon River. This 

 rock is clearly an intrusive dike in" the slates. The edges of the slates next 

 to the dike are much shattered and broken, as the result of this intrusion. 



In the Minnesota" reports these sills are referred to as intrusions in the 

 Animikie slates, but are included in the description of the Animikie which 

 corresponds to our Upper Huronian Series. 



Lawson** assei'ts that these sills are all intrusive, and fortifies his state- 

 ment by proof which seems to be unquestionable, as the following quota- 

 tions of the summary of his argument from his paper will show : 



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 geolog-ical units, not a series of overlapping sheets. 



2. They are flat with uniform thickness over areas more than 100 .square miles 

 in extent, and, where inclined, the dip is due essentially to faulting and tilting. 



3. There are no pj^roclastic rocks associated with them, 

 rt. They are never glassy. 



5. The}' 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. 



11. They are intrusive sills, because of the combination of the following facts: 



1. They are strictly analogous to the great dikes of the region: (a) In their 

 general relations to the adjacent rocks, and in their field aspect; {!>) in that both the 

 upper and lower sides of the sheets have the facies of a dense aphanitic rock, which 

 grades toward the middle into a coai'sely crystalline rock. 



2. They have a practicallj^ uniform thickness over large areas. 



3. The columnar structure extends from lower surface to upper surface, as it 

 does from wall to wall in the dikes. 



4. Thej' intersected the strata above and below them after the latter had been 

 hard and brittle. 



5. They may be observed in direct continuity with dikes. 



6. They pass from one horizon to another. 



eGeol. and Nat. Hist. Survey of Minnesota, Final Kept., Vol. IV, 1899, pp. 476;- 487-488. 

 ^Lawson, loc. cit., p. 29. 



