314 THE JOURNAL OF GEOLOGY. 



and on the Isles of Nipigon Bay are numerous places where Keweenian 

 strata are capped by thick sheets of trap, identical with those which cap the 

 Animikie, but, though these sheets cannot be traced in absolute continuity in 

 the interval, there are many outlying patches which fill the gap. The same 

 trap sheets are found in several instances to pass from the Animikie to the 

 Keweenian, and there are the same evidences of intrusion of independent 

 trap sheets in the Keweenian that are in the Animikie. These rocks are, 

 therefore, of post - Keweenian age, and, to discriminate them from the 

 Keweenian and Animikie, they are designated the Logan sills. 



Coinmeftis : — The fact that all the trap sheets of the Animikie studied by 

 Lawson are intrusive is no evidence that in other areas, not studied by him, 

 there may not be contemporaneous volcanics. The traps in the Triassic of 

 Connecticut and New Jersey are an illustration of this point, a part of them 

 being extrusive and a part intrusive. Also in the Penokee series, the equiva- 

 lent to the Animikie series, while for the main part of the area there are no 

 contemporaneous volcanics, in the eastern end of the series there suddenly 

 appears a great thickness of contemporaneous volcanic fragmentals, and such 

 may occur in the Animikie in the areas not yet studied. 



The inclination of the Animikie series was fully recognized by Irving and 

 Ingall, and this it was which led them to make their estimates of the thick- 

 ness. The statement, that the strata have been dislocated by a great system 

 of faults, may be true, but in the paper it is not supported by any evidence ; 

 and, until detailed evidence is presented, the conclusion of Irving and Ingall 

 as to the thickness seems more probably true than the hypothesis of numerous 

 faults. 



Because the sills are later than the Animikie and Keweenawan strata 

 which they have intruded, is no sufficient evidence that they are post - Kewee- 

 nawan. The thickness of the Keweenaw series is so great that it is quite 

 reasonable to expect that correlative with the later extrusions were intrusions 

 between the older Keweenawan strata. To explain all the facts cited on the 

 northwest coast it is only necessary to suppose that the upper part of the 

 Keweenawan has been removed by erosion, and that the sills now composing 

 the upper layers in these places were overlain at one time by higher mem- 

 bers, which have subsequently been removed by erosion. This is not a violent 

 suppositition, for it is well known that erosion and volcanic extrusion alter- 

 nated many times in single areas during Keweenawan time. 



C. R. Van Hise. 



