Basic Rocks of Northeastern Maryland. — Leonard. 139 
of quartz noryte and olivine noryte was collected but these 
varieties are rarely found. Still nisore interesting is the oc- 
currence of a saussurite noryte in which the feldspar is com- 
pletely altered to zoisite and the hypersthene partially changed 
to fibrous hornblende. A porphyritic variety with large phen- 
ocrysts of diallage occurs at the Mount Hope church. With an 
increase in the amount of diallage the noryte passes over into 
hypersthene-gabbro. While this rock is in general more com- 
pact and somewhat darker in color than those just described, 
it often bears a close resemblance to them. The diallage is 
seldom more abundant than the orthorhombic pyroxene and the 
latter is always present in large amount. 
If compared with the diorytes occurring along the southern 
border of the area, the hypersthene-gabbros, as well as the 
norytes, are seen to be noticeably finer in grain. Where the 
gabbro outcrops at some points, notably along Octoraro creek 
and on the Susquehanna, it is seen to have been considerably 
altered and bears slight resemblance to the original rock. The 
plagioclase is no longer bright and transparent, but is changed 
to a dull, opaque saussurite, while the pyroxene has also altered 
into a greenish, fibrous hornblende. The resulting rock, which 
is a typical saussurite gabbro, has a light greenish color and a 
mottled appearance due to the white and green minerals com- 
posing it. Olivine gabbro is of rare occurrence in this region, 
only a few specimens being found. 
By the alteration of the pyroxene to secondary hornblende 
the hypersthene-gabbro has in some cases been changed into 
gabbro-dioryte or metagabbro. But this metamorphic process 
does not appear to have gone on very extensively and these 
secondary diorytes are comparatively rare. 
Numerous diabase boulders were observed just east of Ris- 
ing Sun, and at various points near the railroad between that 
locality and the state line. These are doubtless derived from 
the trap dike traced by H. C. Lewis through southeastern 
Pennsylvania to the Maryland boundary, which it crossed not 
far from Sylmar station.''' The rock is very fine-grained and 
compact. 
Pyroxenytes and peridotytes occur in dikes and lens-shai)cd 
masses in the noryte and gabbro. The former is also abundant 
♦American Philosophical Society, May l.j. 1885, 10 pp. 
