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ment. On one point Irving seems not to have fully drawn the conclusion which legitimately followed from his observations. This, however, does not invalidate the observations in any way, nor lessen the strength of the many important conclusions which were reached. The only particularized notice by Dr. Lawson of these supposed errors is in reference to the thickness of the Keweenawan. The statement that Irving overestimated this thickness tenfold certainly needs additional justification. The thicknesses given are maxima for the particular region. Irving was perfectly well aware that the Keweenaw series varies greatly in thickness from place to place, being largely of volcanic origin. He also knew that it varies from its maximum thickness to entire disappearance at a not very remote distance from the Lake Superior basin. That a volcanic series is not of great thickness in one particular area of a region is no evidence that it is not so in other parts. If the anorthosite division of the Keweenawan constituted a mountainous mass for the central part of the Minnesota coast, and the upper Keweenawan beds were deposited against them, as before suggested, these later beds may have a very great thickness remote from the anorthosites, or at the inaccessible base of the past mountain range, and this would be quite in accordance with a moderate thickness near the tops of the anorthosite domes.

Lawson^[1] describes the laccolitic sills of the northwest coast of Lake Superior. The trap sills are mainly diabases, but they occasionally pass into gabbros. It is held that there are no contemporaneous volcanic rocks in the Animikie group, and that the trap sheets are intrusive in their origin, rather than subsequent volcanic flows, for the following reasons: They are simple geological units, one not overlapping another; they have a uniform thickness over areas more than 100 square miles in extent; where inclined, the dip is due to faulting and tilting; they have no pyroclastic rocks associated with them; they are not glassy nor amygdaloidal; they show no flow structure, or other distinct properties of effusive rocks; their contacts with the slates are sharp; they never repose upon a surface which has been exposed to weathering or erosion; they are analogous to the great dikes of the region in all their relations; they may be observed in direct continuity with dikes; they pass from one horizon to another; they have a columnar structure extending throughout theur thickness; apophyses pass from the main sheets into cracks of the slate above and below; they locally alter the slates above and below them.

The Animikie strata have been dislocated by a great system of faults, the orographic blocks having been frequently tilted. The non-recognition of this prevalent tilted structure has led to very excessive estimates of the thickness of the series by Irving and Ingalls. In the vicinity of Black Sturgeon River