i62 The American Geologist. September, looi. 
•chroic and was rather poor in iron. So far as known this is 
the only occurrence outside of Maryland of the olivine-hyper- 
sthene rock. 
It has already been stated that these peridotytes alter very 
readily into serpentine and many of them have under- 
gone a more or less complete change into that rock. They 
doubtless represent the original type whence were derived the 
various serpentine masses of the region. There is good reason 
to believe, however, that some of the serpentine has been 
formed from the pyroxenytes. 
SERPENTINE. 
The main portion of the serpentine area lies north of the 
Maryland line in Chester and Lancaster counties, Pennsylvania. 
It is known as the "State Line serpentine" and forms a belt with 
a length east of the Susquehanna river of about seventeen miles 
and an average width of one mile. It also extends across the 
river into Harford county, where it has a length of sixteen 
miles and a maximum width of ^woi miles. The southern 
boundary of the area follows the Mason and Dixon line closely 
as far west as Octoraro creek where it crosses over into Mary- 
land and extends in a southwesterly direction to the Susque- 
hanna and beyond. Besides this main mass there is a small 
area west of Conowingo creek, forming an extension of a 
larger belt in Lancaster county. There is also a smJall outcrop 
* of serpentine, representing a dike, along the railroad just south 
of Conowingo. 
A study of these rocks, especially in their less altered con- 
dition, reveals the presence of two varieties. The first contains 
■crystals of pvroxene imbedded in a compact and quite pure ser- 
pentine. Where the change is not yet complete some of the 
original olivine can still be seen. This tvpe has plainly been 
derived from the peridotyte. 
The second variety has its origin from the pyroxenytes. If 
is possible to trace this change through its different stages from 
the fresh pyroxene rock to the final serpentine product. The 
Tiypersthene and diallage first alter into fibrous hornblende, the 
change commonly beginning on the outside of the different in- 
dividual grains and extending inward, usually along cracks 
if these are present. This process goes on until no trace of 
