i68 The American Geologist. September, i90i. 
in the amount of hornblende and a corrcs])ondin,^ decrease in 
the biotite, the tonalyte passes into a quartz-dioryte. On the 
other hand, liy a decrease in the amount of hornblende and a 
corresponding- increase in the biotite it passes over into the 
biotite-granitc of the Rowlandsvillc area. This latter rock, as 
shown on a later page, is chemically very closely related to the 
diorvtes and can in fact be considered as a biotite-dioryte. 
These intermediate varieties between the granites and the true 
diorytes are of common occurrence and cover considerable ter- 
ritory. 
There is also a change in the feldspar as the rock becomes 
poorer in silica and richer in hornblende. In the biotite-granite 
the feldspar is oligoclase along with a little orthoclase ; in the 
tonalyte it is an acid labradorite, while in the true diorytes this 
constituent is represented by an acid bytpwnite. Nowhere was 
any line of contact observed between the granites and diorytes, 
and careful search failed to reveal the presence of any eruptive 
contact, such as that described by Grimsley along Octoraro 
creek in the vicinity of Porter Bridge. The granite does con- 
tain, it is true, the dark, fine-grained, granitic segregations 
such as those mentioned by that writer. But no reason was 
found for regarding some of these more basic patches as segre- 
gations and some as inclusions. Nor did these change in ap- 
pearance or composition as the dioryte border is approached, 
and no increase in their number or size was observed in that 
direction. In the absence of any better evidence of an eruptive 
contact than is afiforded by these basic segregations, which do 
not resemble inclusions of dioryte or o-abbro, there would seem 
to be slight ground for supposing there is here any such con- 
tact. Careful search failed to reveal the presence of any dikes 
or apophyses of either rock. 
GEOLOGICAL RELATIONS OF THE DIORYTES, GABBROS AND NORYTES. 
Bordering the diorytes on the north and closely associated 
with them are the norytes and hypersthene-gabbros. It was 
found possible to distinguish two kinds of hornblende-feld- 
spar rocks, one original and commonly carrying considerable 
quartz, the other secondary and produced by the alteration of 
the gabbro. In the latter (metagabbro or gabbro-dioryte), 
quartz was usually absent or present only in small amount. 
