STRUCTURAL GEOLOGY. 59 



Structural peculiarities of the gabljro.] 

 The Logan sills. 



penetrates in a confused manner the red rock, with which it alternates both 

 structurally and areally. Large bosses and areas of the red rock, frequently granitic, 

 penetrate the gabbro, and send dikes into it, the dikes sometimes running for several 

 miles from the parent mass. According to Mr. Elftman, gabbro dikes are not known 

 to cut the red rock, but this may be attributed, in a measure, to the assignment by 

 him of such dikes to a supposed later intrusion of diabase, since the gabbro presents 

 petrographic characters resembling those of diabase, when it acts as an intrusive in 

 other rocks. The Beaver Bay diabase, which is considered to be the first great 

 How-stratum from the parent gabbro, carries isolated pieces of red rock, as shown in 

 the chapter on Lake county. At Duluth, while the gabbro in the main lies under 

 and involves the red rock, there are stringers and isolated red rock areas in the gabbro. 

 The appearances, both at Duluth and at Beaver bay, are explicable by supposing the 

 gabbro and the red rock were simultaneously in a state of mobility, allowing at the 

 same time for the greater liquidity of a basic magma under complete fusion, and for 

 the greater penetrating capacity of the acid magma, in a condition of aqueo-igneous 

 fusion or plasticity. 



Tin' ftrui-l in-iil peculiarities of the yabbro. The Loyau sills. Dr. A. C. Lawson 

 gave this name to the great sills which, along the international boundary and thence 

 to Thunder bay, lie in the bedding planes of the Animikie and greatly increase its 

 aggregate thickness.* That these sills are cotemporary offshoots from the gabbro 

 mass is sufficiently demonstrated by their mutual proximity and their similar petro- 

 graphic characters. One of the greatest sills known is that of mount Reunion (No. 

 2064). It has a thickness of about 100 feet, and crowns the summit of a bluff of 

 Animikie, facing north, which rises somewhat over 400 feet above the level of Rove 

 lake. In this rock the augite is of two periods of development, a character not 

 uncommon in the gabbro. The greater part of it is in rather small roundish grains 

 that preceded the plagioclase, but there are a few larger grains that followed the 

 plagioclase, since they embrace the plagioclase ophitically. Several instances occur 

 of the inclusion of olivine in the augite. A little quartz, evidently of secondary 

 origin, is in the angular spaces between the plagioclases, and also is in granophyric 

 intergrowth with a feldspar which itself also appears to be of secondary origin. In 

 several other instances (Nos. 296, 297, 300, 308, etc.) the same petrographic characters 

 have been found in these greater sills, and in one or two of the great dikes that are 

 probably cotemporary with the gabbro, at and near Carlton, at the extreme south- 

 western limit of the gabbro. Still, it appears that in many instances and this seems 

 to be the case when the dikes and sills are narrow the augite is wholly later in 

 date than the plagioclase, and displays only the ophitic relation. It is to be inferred, 



"Bulletin viii, p. 48,1893. 



