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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 Keweenawan 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 maximum 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 Lawson's article takes exception to the author's small estimate, and prefers to accept Irving's figures, until these are proven inaccurate 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 eruptive series, may vary in different areas, but Irving believes that the following "broad horizons" may be recognized: (1) a succession 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-

  1. ↑ l. c., p. 24.
  2. ↑ Geol. and Nat. Hist. Survey of Minn., Bull. No. 8, p. 21.
  3. ↑ Jour. of Geology, Vol. I., p. 312.
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