DAE 
JOURNAL OF GEOLOGY 
»~ JANUARY-FEBRUARY, 1895. 
THE BASIC MASSIVE ROCKS OF THE LAKE 
SUPERIOR REGION. 
IV. THE PERIPHERAL PHASES OF THE GREAT GABBRO MASS IN 
NORTHEASTERN MINNESOTA.* 
C. The Granulitic Gabbros. 
THE granulitic gabbros include a large number of different 
phases characterized by the same general type of structure, but 
differing from one another in mineralogical composition. Olivin- 
itic _ rieties occur in small quantity. In these the constituents 
are fj) : same as those composing the great mass of the normal 
gabb >, viz., magnetite, olivine, diallage and plagioclase. The 
rarity of olivine in the granulitic rocks is in marked contrast with 
its abundance in the gabbroitic variety. In the latter, if one of 
the components is absent it is usually the diallage. In the granu- 
litsc phase, olivine is more frequently absent than present. By 
its loss diallagic varieties result. But pure diallagic phases of 
the granulitic rocks are also uncommon, for in the production of 
the granulitic structure there is a tendency to the formation of 
biotite and hornblende. Thus biotitic and hornblendic varieties 
are much more common among the granulitic gabbros than 
among those with the true gabbro structure. Another quite 
widely spread constituent in the granulitic rocks is hypersthene. 
In these, as in the non-feldspathic gabbros, this mineral appears 
to take the place of olivine. 
‘Continued from this Journal, Vol. II., p. 825. 
WOii6 JOU IN@s its I 
