ORTHOCLASE-BEARING GABBRO. 51 
not only because of secondary quartz, but also because of the nature of the 
feldspar and of the comparatively small amount of the augitic ingredient. 
The orthoclase of these rocks is always more or less clouded, and is 
often reddened and charged with secondary quartz. The two latter results 
of alteration, however, vary greatly in the amount present, and are at times 
nearly or quite wanting, or at least the reddening is diminished so as to be 
hardly perceptible in the thin section, while the quartz may be wholly 
absent. The orthoclase appears always to be later in origin than the pla- 
gioclase, since its contours are always molded around those of the latter 
mineral. 
The plagioclastic ingredient of these rocks gives in some sections angles 
higher than the limit set down for oligoclase, but it never reaches very high 
upon the labradorite range. It is always much less fresh than in the rocks 
of the previous class, in which extreme freshness of the feldspars is the rule, 
and is often reddened as much as the accompanying orthoclase. In some 
cases both it and the orthoclase show an alteration to a greenish chlorite. 
‘The lineation in the polarized light is at times very faintly seen, owing to 
decomposition, but is usually sufficiently distinct for measurement. 
The magnetite of these rocks appears always to be much more highly 
titaniferous than that of the ordinary gabbros. It is generally in relatively 
large fragments, which have at times a marked pinkish tint, and then the 
attraction by the magnet is very feeble. In the thin section it is often 
accompanied by the peculiar white alteration-product indicative of the 
presence of titanic acid. It appears, however, always to be rather a very 
highly titaniferous magnetite than a true titanic iron. In the sections of 
the coarse orthoclase-gabbro of Duluth, the large, irregular areas of this 
ingredient are commonly surrounded by a brownish film, which appears 
usually to be merely ocherous, but is at times certainly made up in part 
of scales of biotite. In some of these rocks the titaniferous magnetite 
forms bunches of such size as to have attracted attention as an iron ore, 
as, for instance, at Duluth, and again at several points in the interior back 
of Grand Marais and Beaver Bay. From some of this ore obtained some 
'Compare G. W. Hawes, in Geology of New Hampshire, Vol. II, Part IV, Plate XI, Fig. 6. 
