THE BASIC MASSIVE ROCKS OF THE LAKE SUPERIOR 



REGION. 



IV. THE PERIPHERAL PHASES OF THE GREAT GABBRO MASS OF 

 NORTHEASTERN MINNESOTA. 1 



A. Introduction. 



In 1886 Professor J. W. Judd 2 described a series of basic 

 rocks from the Inner Hebrides and the adjoining mainland of 

 Scotland and Ireland, that in mineral composition are closely 

 allied to the gabbros with which they are associated. Their 

 structure, however, is quite peculiar. The most notable differ- 

 ence between it and the structure of a normal gabbro lies in the 

 character of the olivine and diallage constituents. These have 

 "more or less rounded outlines, and are imbedded in a plexus of 

 lath-shaped crystals of feldspar ; in polarized light these grains 

 are seen not to be parts of one large crystal, but to have very 

 different orientations. The form of the individuals of pyroxene 

 and olivine at once recalls the structure seen in the granulites" 

 (p. 68). It was therefore called by Judd the " granulitic 

 structure." True gabbros, with the granitic structure, were 

 found only in the central portions of great bosses and flows. On 

 the edges of flows and along their upper and lower surfaces the 

 granitic structure gives place to the ophitic or the granulitic 

 structures. The best examples of the last two types were found : 

 the first in dike rocks ; the second in lava flows. Hence the 

 conclusion was reached that "the only type of rock absolutely 

 characteristic of intrusive rocks is the granitic ; but ophitic 

 varieties and varieties with skeleton crystals in their base abound 

 in, though they are not confined to, intrusive rocks ; while rocks 

 of granulitic structure and those with short and rounded micro- 

 lites in their groundmass are especially abundant among the 



1 Quart. Jour. Geol. Soc, February, 1886, p. 49. 



2 Continued from Vol. I., p. 716. 



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