i64 The American Geologist. September, i90i. 
from Angel island in San Francisco bay, which has originated 
from a rock composed Wholly or mainly of diallage. In this 
instance the alteration seems to have been directly to serpen- 
tine and not through the intemiediate hornblende stage. Other 
examples of serpentine derived from pyroxenyte rocks are men- 
tioned by Drasche''' from the Tvrol and Ireland ; by Berwerthf 
in serpentines near Rosignano and Castellina Maritina, south 
of Pisa, Italy, and by several other writers. 
The following analysis (VI) of a deep green, translucent 
variety of serpentine from the old Broad Creek quarry in north- 
eastern Harford county was made some years ago by F. A. 
Genth of the University of Pennsylvania.:}; 
VI. 
Si02 40.06 MgO 39.02 
AI0O3 1.37 H2O 12.10 
CrsOs 0.20 FegOi 8.02 
NiO 0.71 
FeO 3.43 100.00 
MtiO 0.09 Specificgravitv 2.668 
VEINS AND DIKES. 
The basic and ultra-basic rocks of northeastern Maryland 
are cut by numerous pegmatyte dikes and quartz veins. In ad- 
dition to these there is the diabase dike already mentioned. 
The latter extends in a southwest direction through southeast- 
ern Pennsylvania and into Maryland, probably crossing the line 
in the vicinity of Sylmar. Blocks of diabase were found at 
various points south of the railroad between the latter town and 
Rising Sun, and though no outcrops were observed the bould- 
ers were in general distributed along a line having the same 
course as the dike where it was traced farther north. Grims- 
ley states that south of Liberty Grove a diabase dike was traced 
for nearly a mile, and it is probable that this is part of the same- 
one. H. C. Lewis gives- the age as late Triassic or early 
Jurassic, since in Pennsylvania it cuts through shales of the 
former age, and must, therefore, have been formed at the end 
of that period or at the beginning of the next. 
Cutting the norytes south of Conowingo and occurring also 
at other points are dikes of peridotyte and pyroxenyte. One 
*T. M. p. M., Vol. T. p. 10, 1871. 
fT. M. P. M.. Vol. IV, p. 238. 1870. 
JGeological Report of the Md. "Verde Antique" Marble and other Min- 
erals on the land.s of the Havre Iron Co., in Harford County, JId., by Prof. 
F. A. Genth. 187.5. 
