146 
The American Gcoloo;ist. 
September, 1901. 
clase. An analysis of a somewhat more basic variety of the 
same rock from an outcrop on Octoraro creek at Porter 
Bridge, is given under II. It differs from the other dioryte 
chiefly in structure and is typical of the rock occurring in the 
vicinity of Porter Bridge. Ill is an analysis of a hornblende- 
dioryte poor in quartz from three-quarters of a mile northwest 
of Rising Sun. It is composed of bytownite and hornblende 
with a little quartz, magnetite, apatite and titanite (sphene). 
This rock contains less quartz and is therefore somewhat more 
basic than the true quartz-diorytes of the area. This analysis 
(III) agrees closely with that of a dioryte from Thuringia 
which Rosenbusch* gives as an example of a dioryte poor in or 
free from quartz. The Thuringia rock is likewise composed 
of bytownite and hornblende. 
I. II. III. 
Si02 58 57 55.16 44 04 
Ti02 1.41 .64 2.24 
Zr02 09 .02 .10 
AI2O3 16.10 17.51 20.01 
V2O3 02(018) .04(036) .05(053) 
CroOs uone trace none 
FeoOa 2.89 2.62 4.22t 
FeO 6.12 5.83 8.61t 
NiO, CoO none .01 .01 
Mn 18 .15 .28 
CaO 7.39 8.50 11.68 
SrO trace trace none 
BaO trace trace none 
MgO 2 33 4.35 5.01 
K2O 1.01 1.08 .15 
Na20 2.11 1.83 1.24 
Li20 trace trace trace 
H2O below 105° C. .21 .18 .11 
H2O above 105° C. 1.27 2.01 1.90 
P2O5 37 .21 .52 
FeS2i trace §.30 (02 S) .25(135 8) 
CO2 none none none 
CI nndet. iindet. undet. 
Fl undet. undet. undet. 
100.07 100.17 100.42 
Sp. gr 2.890 2.902 3.037 
*Elemente der Oesteinslehre. 1808. p. 140. 
t Subject to correction for int3uence of po.ssible pyrrhotite. 
tSulphiir calculated as FeS^, but exists as pyrrhotite or other sulphides 
soluble in HCl. 
§l'erhaps mainly pyrrhotite 
