Basic Rocks of Northeastern Maryland. — Leonard. 163 
the original pyroxene is left and a rock is produced consisting 
wholly of secondary hornblende. This transformation may 
go no further and we will have left the smaragditic and gray 
hornblendic rocks found so abundantly throughout the region 
in association with the non-feldspathic rocks. But the altera- 
tion does not always^ stop here, for these amphibole rocks may 
themselves pass over into serpentine. This change was clearly 
•exhibited in a large boulder found near Svlmar. Even in the 
hand specimen this was seen to be composed partly of gray, 
fibrous hornblende, and partly of compact green serpentine, 
the former mineral graduating insensibly into the latter. Under 
the microscope the secondary hornblende, with its rather high 
interference colors and resemblance in all respects to that so 
•often observed surrounding the pyroxene, was seen to be asso- 
ciated with a fibrous aggregate of serpentine. The latter did 
not have the appearance so characteristic of this mineral when 
it is derived from olivine, but resembled the variety produced 
from hornblende. The fibrous aggregates of serpentine sel- 
dom have any regular arrangement, but form a confused mass 
traversing the hornblende substance, from which they can be 
distinguished by their low interference colors of pale blue and 
gray. Occasionally the fibres or columns of hornblende are 
seen separated by the serpentine into which they merge along 
their borders. It is apparent that in this instance the serpen- 
tine is not derived directly from the pyroxenyte, but that this 
is changed first into fibrous hornblende and the latter subse- 
quently alters into serpentine. 
A similar transformation has been described by F. D. Ches- 
ter as occurring in the extension of this same belt of rocks in 
Chester county, Pennsylvania.* 
Many examples might be cited of serpentines formed from 
peridotytes, and in fact the latter are regarded as the chief 
source whence these rocks have been derived. On the other 
hand, it is comparatively rare to find cases where the serpentine 
has undoubtedly come from pure pyroxene rocks, as is the case 
with some of those in northeastern Maryland and adjoining 
portions of Pennsylvania. Chesterf found that many of the 
state line serpentines have been derived from pyroxenyte, which 
►often contains a little olivine. Ransome:}: describes a serpentine 
*Penn. Geol. Surv. Ann. Rept., 1887, p. 104. 
tPenn Geol. Surv., Ann. Rept., 1887, pp. 102-105. 1889. 
tI3ul. Dept. Geol. Univ. of Cal. Vol. I, p. 222, 1894. 
