Basic Rocks of Northeastern Maryland. — Leonard. 153 
less altered to zoisite and is often wholly changed into saus- 
surite. In this regard it differs from the plagioclase of the as- 
sociated pyroxene rocks which is as a rule very fresh and unal- 
tered. 
It is not necessary to add anything more to the description 
already given of the diallage and hypersthene. The former 
mineral is not present in large amount and while it varies con- 
siderably it is rarely more plentiful than the orthorhombic 
pyroxene. The former is by far the most important of the ferro- 
magnesian constituents occurring in these basic rocks. !Magne- 
tite is seldom absent and apatite, while frequently found, is by 
no means abundant. 
Olivine is rare in the gabbros and norytes of this region, 
having been observed in only two specimens, one from the 
small area northeast of Elkton, the other from near Oak Grove 
schoolhouse, one and one-half miles northwest of Rising Sun. 
Metaniorphic clianges in tlie gahbro. The gabbro of this 
area has undergone considerable alteration which has resulted 
in the change of its feldspar into saussurite and its pyroxene 
into hornblende. The metamorphosed rock thus nroduced is a 
typical saussurite gabbro. The saussurite is found under the 
miscroscope to be composed almost wholly of crystals and ir- 
regular grains of zoisite. Some good sized patches of the 
latter gave no evidence, even with the highest power, of being 
formed of small individuals. On the other hand, the larger 
masses are frequently seen to be not single crystals but aggre- 
gates made up of separate grains and crystals. Prisms of 
zoisite without terminal faces are quite common. They show 
the perfect cleavage parallel to the vertical axis, the cross joint- 
ing at right angles to this, the parallel extinction and the biaxial 
interference figure with very small optic angle. The mineral 
is colorless, has a high index of refraction, weak double re- 
fraction, and is optically positive. In thin sections the saus- 
surite is as a rule quite clear and transparent, since the zoisite 
composing it is in good sized grains and crystals, but occasion- 
ally it becomes opaque. As already stated, the pyroxene of 
these gabbros has been changed into fibrous hornblende. The 
two chief constituents of the saussurite gabbro are therefore 
secondary hornblende (smaragdite) and saussurite. Some 
chlorite and a little secondary quartz are commonly present. 
