THE BASIC MASSIVE ROCKS, ETC. 8 I 7 



prevailing during the period of its cooling, produced the 

 coarse-grained gabbro that covers so many hundreds of miles 

 in northeastern Minnesota. Under certain conditions this same 

 magma yielded the very basic rocks here described, but these 

 form such a small mass as compared with that of the great 

 gabbro, that it does not seem wise to designate them by any 

 name that will not at once indicate their very close relationship 

 to the gabbro. Peridotite as the name of a group of rocks pro- 

 duced by the slow cooling of a very basic magma is well enough, 

 but as a name for the aggregation of basic minerals from a com- 

 paratively acid gabbro magma it would be as much out of place 

 as the use of the same name for the olivine-diallage concretions 

 of a basalt. 



The rocks of this class occur, as has been said, along the 

 north boundary of the gabbro area, where they are found alter- 

 nating with the granulitic gabbros of the several varieties. The 

 thickness of the bands ranges from twenty or more feet down to 

 a fraction of an inch only. In a single thin section may some- 

 times be discovered several of them. The line of separation 

 between the bands is nowhere very distinct, and in the thin sec- 

 tion it is seen to be quite gradual. Many times the rocks are so 

 charged with magnetite as to have suggested their being worked 

 for ore. Professor N. H. Winchell, 1 in his discussion of the iron 

 ores of Minnesota, classes this magnetite with that occurring in the 

 normal gabbro under the group name "gabbro-titanic-iron group." 

 The difference between the magnetite of the quartzose beds under 

 the gabbro and of the basic rocks associated with the gabbro is 

 recognized. 



A specimen of one of the basic rocks was submitted to 

 Dr. Hensoldt 2 for microscopic study. In the section exam- 

 ined basic and acid bands occur alternately. The basic portion 

 of the rock (M. 1339), which is from the Akeley Lake region, is 

 described as a compact lherzolite, chiefly composed of yellowish- 



1 The Iron Ores of Minnesota, Bull. No. 6, Geol. and Nat. Hist. Survey of Minn., 

 pp. 123-126. 



2 lb., p. 127. 



