40 The Amert'can Geoloyisf. January, 1895 
liams had been vapidl}^ gathering materials for a systematic- 
work on the massive rock types of Nortli America. Prepara- 
tory to this special attention had been given to the eruptives- 
of the Piedmont plateau, particularly in Maryland. Care- 
fully made field and petrographical studies Avere instituted in 
different areas. Some of the results had already been pub- 
lished.* Two other memoirs were in press, both by the U. S. 
Geological Survey; one on the old volcanics of South Moun- 
tain (which, however, is out of the district just named) by 
Dr. F. Bascom, and the other on the granitic rocks of Mary- 
land by professor Williams and the writer. Still other results 
were nearly ready for presentation. 
As a part of the work outlined there has recently appeared 
the Granites of Cecil County in Kortheastern Maryland,^ by 
Dr. George P. Grimsley. It is to some points brought out in 
this contribution to Mar3^1and geology that attention is di- 
rected. 
The rocks, under consideration are the granite-gneisses 
which are widely known as the Port Deposit granites. They 
are regarded as igneous in origin, though now more or less 
squeezed, but the proofs of their eruptive character need not 
be reiterated here. The area is an extensive one and takes its 
name from the town of Port Deposit, in the neighborhood of 
which are large quarries. From them a large amount of stone 
has been shipped to nearly every part of the United States. 
The rock itself is admirably exposed for a distance of fully 
ten miles on both sides of the stream. The stone is a light 
colored, somewhat gneissoid, biotite granite, which is rather 
coarse grained but seldom shows a porphyritic facies. A more 
or less distinct banding of the light and dark constituents is 
quite characteristic. Both observations in the field and 
microscopical examinations of thin sections indicate clearly 
that the parallel arrangement of the components has been 
secondarily acquired through enormous pressure. 
The area is bordered on the north by trappean gabbro, on 
the east and west by the Piedmont gneisses. The area is di- 
vided medially by the Susquehanna river, which has cut a 
*Williams: IT. S. Geol. Sar., Bui. 28. Washiiiglon, 1880. Also Ameki- 
CAN Geologist, vol. vi, pp. 3.5-49. Minneapolis, 1890. 
f. Tour. Cincinnati Soc. Nat. Hist., vol. xvii, pp. 59-07 and 78-114. 
Cincinnati, 1S94. 
