690 THE JOURNAL OF GEOLOGY. 



cession of interbedded 'traps,' amygdaloids, felsitic porphyries, 

 porphyry-conglomerates, and sandstones, and the conformably 

 overlying thick sandstones, as typically developed in the region 

 of Keweenaw Point and Portage Lake on the south shore of 

 Lake Superior." 1 



Although no distinct line of division between them can be 

 pointed out, the beds of the series naturally fall into an upper 

 division made up wholly of detrital material, principally shales 

 and red sandstones, and a lower division consisting chiefly of a 

 succession of basic flows, layers of conglomerate and sandstone 

 and quite a large proportion of flows of acid eruptive rocks. 

 The thickness of the upper division is estimated at 15,000 feet at 

 its greatest, and that of the lower division at from 22,000 to 

 24,000 feet. 



The recent discovery that the central part of the Keweena- 

 wan is underlain unconformably by a great mass of anorthosite, 

 which along the middle portion of the Minnesota coast comes to 

 the surface in many places, suggests to Lawson 2 that the maxi- 

 mum thickness of the lower Keweenawan beds at this place must 

 be much less than Irving's estimate. His own figures are only 

 about one-tenth those of Irving. VanHise 3 in a review of Law- 

 son's article takes exception to the author's small estimate, and 

 prefers to accept Irving's figures, until these are proven inaccu- 

 rate by careful detailed investigation of the problem in the field. 



Since it is only in the lower division that eruptive rocks occur, 

 our attention will be confined entirely to this. It is not possible 

 to determine positively for the entire series the actual succession 

 of the subordinate members belonging in it, for this, in an erup- 

 tive series, may vary in different areas, but Irving believes that 

 the following "broad horizons" may be recognized: (1) a suc- 

 cession of heavily bedded coarse-grained olivine and orthoclase 

 gabbros, forming the base of the series; (2) a series of olivine 

 diabases and diabase-porphyrites, occurring at the lower hori- 



n. c, P . 24. % 



2 Geol. and Nat. Hist. Survey of Minn., Bull. No. 8, p. 21. 

 3 Jour, of Geology, Vol. I., p. 312. 



