(56 THE GEOLOGY OF MINNESOTA. 



[The feldspar masses. 



and vary in mineral composition, and in the last case the diabase may have gathered 

 up and transported masses of earlier cooled rock. Figure 2 is a contoured map 

 of the summit of Carlton peak from Mr. Elftman's note hook. The masses are isolated, 

 and compose a small portion of the entire hill, some of them being 200 feet lower 

 than others and separated by intervals of the ordinary diabase. 



Within the main gabbro area are known several masses of similar rock, but they 

 are not separable from the gabbro itself. They are phases of the formation resulting 

 from a segregation and concentration of the feldspathic element. So far as observed 

 they are not abruptly contrasted with the surrounding rock in a manner similar to the 

 separation of the feldspar masses, but are associated about their peripheries with more 

 or less of the ferro-magnesian minerals, and seem to grade into the general gabbro. 

 At the same time the gabbro itself consists occasionally of nearly as pure feldspar as 

 that of these isolated masses. When unweathered and coarsely crystalline, such 

 areas do not attract attention, but are readily grouped with the general gabbro mass. 

 The original habitat therefore of these feldspar masses may be assumed to be the 

 gabbro itself. The manner of their formation was suggested to the writer by 

 observing the accumulation of porphyritic crystals in a diabase at and westward from 

 Gunflint lake. A coarsely porphyritic diabase forms a sill in the Animikie at the 

 outlet of Gunflint lake. This sill is continuous westward, and is exposed along the 

 Port Arthur, Duluth and Western railway. After a distance of about a mile toward 

 the west, it was noticed that the feldspar crystals began to group themselves in couples, 

 and in triplets, and then into irregular clusters. It was noted further that these 

 clusters reached the size of a foot and more in diameter, and gave a blotched aspect 

 to the diabase by reason of their lighter color. In traveling over the surface of this 

 sill, which is denuded and at the same time much thickened, still larger areas of 

 feldspar were seen, and it appeared as if numerous foreign masses were included in 

 the diabase. These feldspar masses, however, were not sharply set off by rounded 

 contours from the enclosing diabase, but presented angular projections and enlarge- 

 ments. It became apparent that it only required the continuation of such devel- 

 opment in the oi'dinary diabase to reach the size and the purity of the masses seen 

 at Beaver bay.* 



If, then, it be admitted that these masses were indigenous in the gabbro, it remains 

 onlyto account for their generally rounded contours and their greater number at Beaver 

 bay. Generated in the gabbro mass before the Beaver Bay diabase left it, these masses 

 must have had a tendency, on account of less specific gravity, to rise toward the top of 

 the gabbro. It is quite reasonable to suppose that the minerals of that magma, as they 

 took crystalline form, and if they became thus grouped, would, under unconstrained 



* The generation of the feldspar masses from the general yabbro magma had previously been asserted by A. H. EI.KTMAN. 

 Twenty-second Annual Keporl, p. 178, 1893 [1894J. 



