THE BASIC MASSIVE ROCKS, ETC. 8 1 5 



lavas" (p. 75). The cause of the granulitic structure was 

 thought to be the crystallization of the rock's components during 

 a period before the internal movements of the rock-mass had 

 ceased. The granitic and the ophitic structures occur only in 

 masses of considerable dimensions, where the original molten 

 magma yielding the gabbro or the diabase, existed for some 

 time in a state of perfect internal equilibrium. It is when crys- 

 tallization goes on in a mass that is in actual motion, or one 

 whose portions are in motion relatively to each other, in con- 

 sequence of strains set up in the magma, that a granulation of 

 the augite and olivine results. 



The rocks so ably described by Judd in the article referred 

 to, have their exact counterparts in northeastern Minnesota. 

 The great mass of the gabbro constituting the "basal flow" 

 in this region has the typical granitic structure. Along its 

 northern edge, however, near the bottom of the mass, occurs 

 a series of rudely bedded rocks, that differ so markedly from 

 the gabbro that they have been regarded as distinct types 

 by most geologists who have seen them in place. Many of them 

 are dense, heavy, vitreous-looking rocks, others are fine-grained, 

 dark gray ones with a sandy texture and often a resinous lustre, 

 while still others have the appearance of a very fresh, brilliant 

 magnetite. Although they mark the northern limit of the 

 gabbro area throughout its entire extent, their best developments 

 are in the neighborhood of Akeley Lake, in Sec. 29, T. 64 N., 

 R. 5 W.; along the north shore of Iron or Mayhew's Lake, in 

 Sees. 29 and 30, T. 65 N., R. 2 W.; and Sec. 36, T. 65 N., R. 3 

 W.; in the country between Little Sasaganaga and Gabamichi- 

 gamak Lakes, in the northwest portion of T. 64 N., R. 5. W., 

 and on the north shore of the lake last named, in Sees. 31 and 

 32, T. 65 N., R. 5 W. In the reports 1 of the Minnesota survey 

 these rocks are called by various names, such as muscovado, 

 quartzite, iron ore, and in the field notebooks of the members 

 of the United States Geological Survey, quartzites, silicified 

 gabbros, etc. The mere recital of these names is enough to 



1 15th Ann. Rept., pp. 183, 351, etc. 16th Rept., p. 355, etc. 



