THE BASIC MASSIVE ROCKS, ETC. 699 



density of the varieties depends of course upon their composition ; 

 the larger the proportion of feldspar present the lower the specific 

 gravity. Of the three specimens whose densities were determined, 

 one (10440) was found to have a specific gravity of 2. 8061, 

 another (8786) of 2.9475, and the third (8589) of 3.0636. 



The sections of nearly all specimens taken from the interior of the 

 gabbro area, or from points at some little distance from its north- 

 ern edge are similar, in that they represent a very fresh rock, 

 whose structure is monotonous and whose composition is quite 

 simple. All contain magnetite, olivine, pyroxene and plagioclase 

 as primary constituents, and many have in addition as secondary 

 components, biotite, chlorite and quartz. The proportions of 

 secondary products present are never sufficiently large to affect 

 the characteristics of the rock as a whole, though they be abund- 

 ant enough to change materially its appearance in thin section. 

 The usual succession in the formation of the primary minerals is 

 as indicated, and in this respect does the gabbro of the mass 

 under discussion differ most essentially from the other "gabbros" 

 of the same and neighboring regions, for in all of the latter 

 rocks studied the pyroxene is younger than the plagioclase. 



The feldspar is the most abundant of the essential compo- 

 nents, sometimes constituting, as it does, almost the entire section. 

 It is nearly always in large grains, whose contours are very 

 irregular in shape, and only very rarely resemble those of the 

 lath-shaped grains of diabasic plagioclase. The mineral is quite 

 fresh and is devoid of secondary inclusions, other than a few 

 flakes of kaolin and small flecks of some chloritic substance. 

 The characteristic acicular inclusions of gabbroitic feldspar are 

 sometimes absent from the plagioclase of the Minnesota rock, 

 but more frequently they are present in the usual forms. Small 

 areas of augite and little grains of biotite and magnetite are also 

 enclosed in the feldspar, and dust-like particles are scattered 

 everywhere throughout the grain. The inclusion of augite within 

 the plagioclase would seem to show that the latter mineral is 

 undoubtedly younger than the former ; but certain triangular 

 areas of pyroxene between grains of plagioclase would point to 



