212 TRANSACTIONS OF THE [may 21, 
shown by the grains of the mineral. The magnetite is usually 
separated from the feldspar by a narrow zone made up of small 
scales of deep reddish brown, strongly pleochroic biotite. The 
biotite not only surrounds the exterior of the magnetite grains, 
but where the latter, as often happens, contain cavities filled 
with feldspar, the cavities have a lining of biotite. On the other 
hand, where the magnetite is in contact with augite or olivine, 
the biotite is absent. Quite often a grain of magnetite may be 
seen, one portion of which is enclosed in augite and another in 
plagioclase. In such cases only the latter portion has the zone 
of biotite. An example of this arrangement is shown in the 
figure. Here the two large grains of magnetite and two of the 
smaller ones are enclosed chiefly in plagioclase, but each is in 
contact with augite along a portion of its border. The presence 
of the biotite rim between the magnetite and the feldspar, and 
its absence between the magnetite and the augite, are clearly 
shown. The same thing is repeated in every section. The 
lower part of the figure also shows several small grains of mag- 
netite in a continuous mass of biotite scales. It is plain that 
these grains are the residue of a single large grain of magnetite 
which has been partly destroyed to form the biotite. This, like 
the previous phenomenon, is constantly repeated, and, indeed, is 
