304 The American Geologist. November, i9G5 
of the dyke just described. In microscopic section it shows 
serpentine altered from olivine and secondary tremolite and 
anthophyllite partially altered to serpentine. Enstatite 
may be present fresh or unaltered. Talc, calcite and mag- 
netite occur in abundance. From the fresh structure seen 
under the microscope and the presence of olivine in various 
stages of alteration to serpentine the original rock is de- 
cided to be a peridotyte. 
The third dyke shows one exposure in Montgomery 
county. On the edge of the gabbro area, along Arrowmink 
creek. A second exposure known as Castle Rock, is situated 
in Delaware county near Edgemont. It is an example of 
serpentine resulting from the alteration of a pyroxenyte. 
The rock is medium grained and dark green, composed of 
fibrous tremolite, augite, enstatite and talc. The original 
pyroxenes are augite and enstatite while tremolite is a nec- 
essary product. All three minerals show alteration to talc 
which pierces them in all directions. This description ap- 
plies to a specimen obtained from the center of the mass ; 
in an outlying portion the rock is almost completely ser- 
pentinised and is a dense, massive, green serpentine of uni- 
form character. The rectangular cleavage of augite out- 
lined by iron oxide can be seen in the serpentine. 
The outcrop of the fourth dyke extends sporadically 
from Guelph Mills, in Montgomery county southwest into 
Chester county. Three-fourths of a mile south of Paoli the 
dyke gives rise to a barren ridge which is a distinct topo- 
graphic feature for ten miles to the southeast. The rQck 
south of Paoli is a light yellowish green serpentine very 
massive in character and yet under the microscope showing 
evidence of alteration from olivine and in a small part from 
a pyroxene. 
A study of the field relations and microscopic sections 
of the serpentines, of the Philadelphia belt shows that they 
are secondary to pyroxenytes and peridotytes which they 
have in a large measure replaced. Such an origin has % 
been ascribed to the greater part of the serpentines of the 
world and it is only in a few cases that they have been re- 
garded as secondary products of sedimentary rocks. 
