140 The American Geologist. September, looi. 
along the serpentine border, where it appears to be a basic 
facies of the noryte, into which it merges by intermediate 
varieties. This non-feldspathic type is a coarse-grained aggre- 
gate in which the constituents can readily be distinguished ; 
they are a reddish brown hypersthene with which is associated 
a greater or less amount of greenish diallagc. These pure py- 
roxene rocks are found in the vicinity of Oakwood, along the 
state line north of Rising Sun and at several other localities. 
The peridotytes are likewise coarse-grained and frequently 
have a porphyritic structure. They contain olivine in addition 
to the pyroxenes. The largest mass is near the Mount Hope 
church, one mile and a half northeast of Rising Sun, where the 
rock has the peculiar mottled appearance known as poikilitic, 
due to the dissemination of small grains of greenish olivine 
through large individuals of pyroxene. When the light is re- 
flected from a cleavage surface these grains form dull spots. 
Other localities for these olivine rocks are at the Oak Grove 
schoolhouse, two miles northwest of Rising Sun ; along Oc- 
toraro creek above the paper mill, and beside the railroad just 
south of Conowingo. 
The serpentine borders the noryte-gabbro mass on the 
north. It is known as the "State Line serpentine" on account 
of its occurrence along the boundary between ]\Iaryland and 
Pennsylvania. The greater portion of the mass lies across the 
line in Chester county. The southern margin coincides ap- 
proximately with the line as far west as Octoraro creek, where 
it bends to the south and enters Cecil county. The area has a 
length east of the Susquehanna river of seventeen miles and 
an average width of one mile. Between Conowingo creek and 
the river there is another small mass of serpentine which ex- 
tends across from Lancaster county, Pennsylvania. The region 
over which this rock outcrops presents an uninviting and deso- 
late apeparance, making very appropriate the term '"barrens," 
which is often applied to it. 
PETROGRAPHICAL DESCRIPTION OF ROCK TYPES. 
DIORYTE. 
The diorytes are confined to the southern edge of the area 
where they form a belt varying from one to one and a half 
miles in width, the widest portion being near the eastern end. 
