60 THE GEOLOGY OP MINNESOTA. 



[The Logan sills. 



therefore, that when the gabbro magma consolidated in narrow dikes, or in large 

 surface flows, there was some force that restrained the first generation of augite 

 (contemporary with or earlier than the plagioclase) and only allowed it to form 

 ophitically, after the generation of all the other minerals; but that in some cases the 

 conditions were not sufficiently pronounced and uniform, and, as in many of the great 

 sills, both generations of augite are seen. Hence, the characteristic diabasic 

 structure is an incident of consolidation, and does not denote any difference in 

 character, origin or date of the magma from which it resulted, by which the resultant 

 rock can be differentiated, genetically or chronologically, from the gabbro. Most of 

 the later eruptions of the Keweenawan coming probably from the same source as the 

 sills of the Animikie, being of less volume, took the petrographic characters of 

 diabase, as will be seen by examining the chapters devoted to petrography. 



The Logan sills were at first believed to be, like the surface flows of the 

 Keweenawan, of the same date as the sedimentary strata between which they lie. 

 This was the view of Bell, Irving, and, indeed, of all geologists until quite recently, 

 the writer included; and it was for this reason that on seeing the gabbro occupying 

 a position low down in the " Animikie '' at Chub (Akeley) lake, the age of the gabbro 

 in its commencement was said to be near the bottom of the Animikie.* Although 

 the nature of some of these sills was recognized by a few geologists earlier, especially 

 by Mr. E. D. Ingall, of the Canadian survey.f it was not till 1893 that their true age 

 and geological significance were fully recognized. This was by Dr. Lawson, who 

 stated as below, the evidence that these sills are intrusive within the slates. 



I. The trap sheets associated with the Animikie strata are not volcanic flows, because of the combination 

 of the following facts: 



1. They are simple geological units, not a series of overlapping sheets. 



2. They are flat, with uniform thickness over areas more than 100 square miles in extent, and, where 

 inclined, the dip is due essentially to faulting and tilting. 



3. There are no pyroclastic rocks associated with them. 



4. They are never glassy. 



5. They are never amygdaloidal. 



6. They exhibit no flow structure. 



7. They have no ropy or wrinkled surface. 



8. They have no lava-breccia associated with them. 



9. They came in contact with the slates after the latter were hard and brittle and had acquired their 

 cleavage, yet they never repose upon a surface which has been exposed to sub-aerial weathering. 



II. They are intrusive sills because of the combination of the following facts: 



1. They are strictly analogous to the great dikes of the region : (a) In their general relations to the 

 adjacent rocks and in their field aspect. (6) In that both the upper and lower sides of the sheets have the facies 

 of a dense aphanitic rock, which grades towards the middle into a coarsely crystalline rock. 



2. They have a practically uniform thickness over large areas. 



3. The columnar structure extends from lower surface to upper surface, as it does from wall to wall in 

 the dikes. 



4. They intersected the strata above and below them after the latter had been hard and brittle. 



5. They may be observed in direct continuity with dikes. 



* Sixteenth A n n ual Report, p . 85, 1887 [ 1888] . Notwithstanding this, the first description of an intrusive sill in the Animikie 

 was penned by .1. G. NORWOOD, in his report to D. D. OWES, p. 404. who also gave an illustration . He also described and 

 illustrated in the same report the connection between the dikes and the " crowning overflow." The writer described an Animikie 

 sill in Wauswaugoning bay, in 1878. Xinlh A iiiiual Report, 1881, p. 68. 



t Descriptive sketch of the Physical Geography <tn<l Geology of Canada, 1884, pp. '21, ?1. 



