Basic Rocks of Northeastern Maryland. — Leonard. 149 
Sections in prism zone showed an extinction parallel to these 
lines. 
Pieces cut parallel to the macropinacoid give in converged 
polarized light two optic axes and a bisectrix in the field, show- 
ing that the brachydiagonal axis is the acute bisectrix and that 
the mineral is positive. These optical properties establish the 
fact that this is hypersthene and not enstatite, bronzite or 
diallage. The specific gravity is 3.50. 
The inclusions, to which the hypersthene owes its bronzy, 
metallic lustre, are commonly quite abundant but may be entire- 
ly absent. They do not dififer from those so often described as 
occurring in this mineral. 
Diallage is usually present in the norytes as an accessory 
constituent. It is readily distinguished from hypersthene by 
its lack of pleochroism. It occurs better crystallized than the 
latter mineral, being found in irregular grains which have a 
green color in transmitted light. In a specimen from near the 
Mount Hope church the diallage formed large porphyritic crys- 
tals with a*:prismatic habit, imbedded in a groundmass of hy- 
persthene and feldspar. The specific gravity is 3.28. 
The other accessory constituents are magnetite and apatite. 
The former is not always present and is usually not very abund- 
ant. 
Metamorphic changes in the norytes. For the most part 
these rocks have undergone but little alteration. The most 
noticeable change is the one already mentioned, namely, the 
transformation along certain planes of the pyroxene into horn- 
blende. In the hand specimen these planes appear as narrow 
green bands or lines traversing the rock in various directions, 
often parallel, and when examined under the microscope they 
are seen to have ueen formed by the alteration of the original 
constituents. The lines cross the slide from one side to the 
other, cutting across many individuals both of the pyroxene 
and bytownite. Along these zones the hypersthene has been 
changed into a colorless, or light green, fibrous hornblende. A 
narrow band of this secondary hornblende frequently passes 
through several hypersthene individuals which are elsewhere 
entirely unaltered. The feldspar is also transformed along 
these lines into a greenish mineral resembling chlorite. Oc- 
casionally small patches of zoisite are developed. Where these 
