Basic Rocks of Northeastern Maryland. — Leonard. 173 
peridotytes and pyroxenytes are found associated with the gab- 
bros and norytes, where they have by their alteration produced 
extensive masses of serpentine. 
In both the Delaware and Cecil-Harford areas the basic 
rocks are often rich in quartz. But in the former state this is 
found in the gabbros, while in Maryland it is the diorytes that 
■carry the quartz, the latter mineral being almost absent from 
the norytes and gabbros. It has been found at only two points, 
namely, in the vicinity of Mount Hope, Baltimore county, and 
the junction of Stone run and Octoraro creek. Brown horn- 
blende occurs as an accessory constituent in the other areas, 
but was not observed in any of the rocks under discussion. 
Olivine is equally rare in all three districts. Biotite is found 
in the gabbros near Wilmington but is lacking in the ^Maryland 
rocks. 
The area under discussion differs in certain respects from 
those lying to the east and west of it. One distinguishing feat- 
ure is the abundance of dioryte, usually rich in quartz, and its 
passage into the sfranites. Another is the comparatively slight 
development of gabbro-dioryte from the gabbro, an alteration 
which has taken place on a large scale in the other two areas. 
Though many of the gabbros and norytes show the early stages 
of this change, those rocks are comparatively rare in which the 
pyroxene has been completely altered to hornblende. This 
does not include the saussurite gabbros which have had both 
their original constituents changed into secondary products. 
A third difference is the presence in this area of many large 
granite and pegmatyte dikes in the noryte and serpentine Some 
large blocks of pegmatyte were found at one point in the Balti- 
more area* but this was the only instance of such an occurrence 
within the basic eruptives of that district. 
RfiSUMfi AND GENERAL CONCLUSIONS. 
In northeastern Maryland a series of acid and basic eruptive 
rocks has broken through the ancient gneisses (pre-Cam- 
brian?). The following types are represented: biotite-granite, 
dioryte, quartz-dioryte, quartz-mica-hornblende-dioryte (tona- 
lyte), noryte, hypersthene-gabbro, pyroxenyte, peridotyte, and 
serpentine. The basic and ultra-basic varieties have been cut 
*Bul. U. S. Geol. Survey, No. 28, 1880, p. 2.5. 
