Acidic Ervpfives of Xortheastern Jfari/land. — Keyes. 41 
wide, deep gorge into the massive ciystallines of the region, 
extending from beyond the state boundary nearly to the 
mouth of the stream. For the greater part of the distance 
the river flows in a canyon-like course from 250 to 300 feet 
below the general level of the Piedmont plain. The high 
clitfs and salients of granite stand out prominently on either 
side of the water course and form conspicuous features of 
the picturesque A'alley. 
Under the microscope thin sections of the granite from 
Port Deposit show a more or less distinct parallel arrange- 
ment of the minerals. The quartz is broken and granulated, 
the biotite is not so abundant as in some of the other Mary- 
land granites ; microcline is of common occurrence; e'pidote 
and muscovite are developed in many places from the feld- 
spar ; while small dark colored garnets are not of unfrequent 
occurrence. 
The granite is cut through in a number of places by dioritic 
dikes which vary in width from a few inches to 400 or 500 
feet. Contrary to what has been commonly supposed to be 
the case, the southern part of the area is more gneissic than 
the northern. Consequentl}^ the Port Deposit rocks, which 
may be properly regarded as gneiss, pass by gradual transi- 
tions into the massive granite of Rowlandville, which lies to 
the north. Another feature which has been observed is that 
as the granite approaches the gabbro area there is a greater 
and greater development of the ferro-magnesian minerals, 
until at the contact it often becomes exceedingly difficult to 
determine whether the rock is really a granite or a gabbro. 
Another notable characteristic of the granite is the presence 
of numerous basic secretions, which often have the appear- 
ance of rounded inclusions. 
Two types of granitic rocks have been mentioned. The one 
is near Rowlandville and the other in the neighborhood of 
Port Deposit, the latter being the more gneissic. In the for- 
mer the rocks have been not sufficiently s'queezed to entirely 
()V)literate the original characters. They do, however, show 
in a remarkable manner the effects of orographic pressure 
which has changed both the constituents and the structures 
in a very interesting way. One of the most prominent meta- 
morphic changes in the rock, as fully emphasized by Dr. 
