172 The American Geologist. September, looi. 
to the websteryte when it drops suddenly from 20.01 per cent 
to 1.94 . With diminishing siHca there is also a decrease in 
the alkalies, but a decided increase in the lime. The presence 
of vanadium in these rocks is worthy of notice. Number II., 
which is given by Grimsley as an analysis of a biotite-granite, 
corresponds very closely with the analyses of quartz-mica- 
dioryte and of tonalyte as given by Rosenbusch.* Its rather 
low silica content, its high percentage of lime (4.9) and rela- 
tively low percentage of potash (2.05), together with a large 
amount of magnesia, all seem to indicate a relationship with 
the diorytes rather than the granites. From the point of view 
of its chemical composition it should probably be considered a 
quartz-mica-dioryte rather than a granityte. But what is of 
special interest in the present discussion is the fact that this 
rock lies on the border between the diorytes and granites. 
It will be seen from analyses III., IV. and Y., that there are 
two types of dioryte, a more acid quartz-mica-hornblende- 
dioryte and a basic hornblende-dioryte. The former is poorer in 
silica than the typical tonalyte, though it agrees with the latter 
in mineralogical composition. The hornblende dioryte of 
number V. contains only a small amount of quartz and is some- 
what more basic than much of the true dioryte of the area, 
which is rich in quartz. It will be observed that the tonalytes 
(III., IV.) approach the granite in composition, while the anal- 
ysis of the dioryte proper (V.) corresponds quite closely with 
that of the noryte (VI.), the only essential diiTerence between 
the two being in the magnesia. 
If the basic eruptives of northeastern Mai-yland are com- 
pared with those occurring in the vicinity of Baltimore on the 
one side, and with those of Wilmington, Delaware, on the 
other, certain resemblances and differences are at once appar- 
ent. The area under discussion, occupying as it does an inter- 
mediate position between the two, might be expected to exhibit 
certain features common to both. Perhaps the most marked 
characteristic of the rocks of all three areas is the abundance of 
hypersthene present. This is especially true in northeastern 
Maryland and Delaware, where this constituent plays a more 
important role than the diallage, the latter being present only in 
small amount. In both the Baltimore and Cecil-Harford areas 
♦Elemente der Gesteinslehre. 1898. p. 140. 
