Basic Rocks of Northeastern Maryland. — Leonard. 145 
ence of twinning according^ to both the albite and perichne laws. 
Zonal structure is frequently found but is more or less ob- 
scured bv the alteration which has taken place in the feldspar. 
The plagioclase is quite free from inclusions and differs in this 
respect from that of the gabbros and norytes. 
The alteration of the feldspar to epidote has gone on ver\' 
extensively. All stages of this progress of epidotization were 
observed from that in which the change has only just com- 
menced to that in which the epidote has almost replaced the 
original mineral. At first the plagioclase contains only small 
scattered crystals and the twinning lamellse are still evident. 
As the alteration progresses the epidote crystals become more 
numerous and crowded together ; larger individuals also ap- 
pear, often with no regular crystal outline. The feldspars in 
the sections now appear opaque on account of the accumula- 
tion of minute, closely compacted crystals which fill them and 
prevent the light from passing through. The hand specimens 
show that the feldspars have lost their glassy appearance and 
become dull and white. The crystals of epidote have no defi- 
nite arrangement, but are distributed in an irregular manner 
through the host. In most instances the outer edge of the 
crystal is free from epidote. the latter being confined to the 
inner portions of the feldspar individuals. 
The Jwriiblende, which is compact and green, is, next to the 
feldspar, the most abundant constituent. It seldom occurs in 
well defined crystals, but is usually in imperfectly developed in- 
dividuals or confused aggregates. Occasionally, as in some 
of the diorytes near Porter Bridge, the prismatic faces are well 
developed and give rise to columnar forms which on weather- 
ing stand out upon the surface. Twins are not uncommon, 
the twinning plane being the orthopinacoid. One basal section 
of such a twin showed several twinning- lamellae intercallated 
between the two larger halves. 
The pleochroism is very pronounced and does not differ 
from that usually found in hornblende. 
Quart.-: is an abundant constituent of most of the diorytes. It 
is commonly in good sized grains readily distinguishable in the 
hand specimens and u.sually of a beautiful blue color. The 
larger individuals not infrequently show shattered borders due 
to mechanical deformation, and this granulation mav effect the 
