THE BASIC MASSIVE ROCKS, ETC. 7 1 1 



grains, but sometimes it is included within them. The larger part 

 of the mineral is undoubtedly primary, while a smaller portion is 

 probably secondary. By its alteration it gives rise to biotite, as 

 mentioned above, through reactions set up between it and the 

 contiguous plagioclase, so that often a grain of the magnetite is 

 entirely surrounded by a true reaction rim composed entirely of 

 biotite. Leucoxene decomposition products were not once 

 observed. 



Nowhere in the normal gabbro does the magnetite occur in 

 sufficient amount to constitute an ore, but in certain phases of 

 the rock that have lost entirely the gabbro characteristics, it is 

 known to exist in great quantities. Prof. Winchell 1 describes 

 these ores in detail and gives analyses of them ; but most of the 

 titaniferous magnetites of this author's gabbro-titanic-iron group 

 do not occur in the normal rock of his basal mass. They are 

 found either in its peculiar phases to be described later, or in the 

 Animikie and Keweenawan coarse-grained diabases, whose mag- 

 netite is always highly titaniferous, and in which there is always 

 an abundance of leucoxene. Only a few qualitative tests have 

 been made on the magnetite separated from the gabbro, but 

 they all agree in showing no trace of titanium. If, upon further 

 investigation, it is found that an absence of titanium from the 

 magnetite of the basal gabbro is characteristic for the rock, an 

 important difference will have been discovered as existing between 

 it and the rocks of the interleaved flows of nearly similar compo- 

 sition in the underlying and overlying series. 



The only other original component seen in any sections is 

 apatite. This is in the usual form, as colorless, acicular crystals 

 imbedded in feldspar, and in the various alteration products of 

 the diallage and olivine. It is present only in very small quantity. 



Quartz is rare as a secondary substance, mingled with other 

 secondary products in the most altered phases of the rock. In one 

 section (No. 8796) it is filled with tiny, opaque, acicular inclusions. 



In order to learn something of the limits through which the 

 rock varies in its chemical composition two specimens were 

 ^ull. No. 6. Minn. Geol. Survey, p. 117 and 125. 



"W 



