ANALYTICAL ABSTRACTS. 313 



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 par- 

 ticular 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' describes the laccolitic sills of the northwest coast of Lake Super- 

 ior. The trap sills are mainl}^ diabases, but they occasionally pass into gab- 

 bros. 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 asso- 

 ciated 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 extend- 

 ing throughout their 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 - i-ecognition 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 



' The Laccolitic Sills of the Northtvest Coast of Lake Superior, by A. C. Lawson. 

 In Bull. 8, Geol. and Nat. Hist. Sun, Minn., pp. 24-48. 



