440 FRANK F. GROUT 
minerals interlock across the contact. In most cases there is no 
great difference in the mineral constituents of the bands, but only 
Fic. 1.—The banded gabbro of Duluth, Minnesota. The banding in this outcrop 
is about as conspicuous as in the average. 
Fic. 2.—The bands in this gabbro outcrop are irregular and the color contrast 
is very slight, but the lighter bands are polished by glaciation. 
in the relative abundance of the minerals. The colors of adjacent 
bands may be only slightly different, or in some cases may show a 
strong contrast. In a rock mass containing a variety of minerals 
