136 The American Geologist. September, i-joi. 
held after the preliminary survey of the field. Subseqvient 
work has led to some changes in the boundaries of the gabbro 
and serpentine and has brought to light additional rock types 
which are not represented on the previous map. 
It was later found necessary to discontinue the study of the 
area and not until the autumn of 1897 was the work resumed. 
The region was then revisited under the auspices of the Mary- 
land Geological Survey and the investigations were extended 
beyond the area previously studied. 
GENERAL DESCRIPTION OF THE AREA AND ITS 
ROCK TYPES. 
Location and Extent. The area of basic rocks treated in this 
paper lies within the belt of ancient crystallines which extends 
along the eastern flank of the Appalachian mountains, forming 
what is known as the Piedmont plateau. The formations which 
compose the Piedmont region are generally considered as pre- 
Cambrian and are therefore Archean and Algonkian in age. 
They are separated from the coast by the unconsolidated depos- 
its of the Coastal plain. In Maryland the crystalline region is 
divisible into two portions ; the western half consists of unfos- 
siliferous and only slightly metamorphosed rocks which are of 
undoubted sedimentary origin ; in the eastern division the rocks 
are wholly crystalline and show little or no certain evidence of 
clastic origin except in such infolded masses as the Deer Creek 
quartzytes and Peach Bottom slates. In this eastern portion of 
the Piedmont belt the rocks are chiefly gneisses, schists and 
marble, through which have broken eruptive masses of granite 
dioryte, gabbro, pyroxenyte and peridotyte. It is witli one of 
these areas of eruptive rocks that this paper deals. 
The rocks under discussion form a long, narrow belt ex- 
tending in a general southwesterly direction from a point in 
Cecil county eleven miles west of the Delaware line, through 
Harford and into Baltimore county, to a point fifteen miles 
northeast of Baltimore. It has a length of about thirty-five 
miles and a width varying from two to three miles. The Sus- 
quehanna has cut its channel across this mass of igneous rocks, 
dividing it into two parts. This paper treats especially of that 
portion lying to the east of the river, in Cecil county, embracing 
an area of approximately twenty-two square miles. The types 
