250 H. P. CUSHING—SYENITE-PORPHYRY DIKES. 
the potash. This of course represents only the average make-up of the 
feldspar. Different individuals unquestionably vary much in composi- 
tion. The slight amount ef magnesia in the rock is no doubt due to the 
little chlorite present. The quantity is so small as not to appreciably 
affect the calculation, which must give the composition of the feldspar 
within very slight limits. The constant presence of manganese in the 
analyses is of interest, though its source is purely a matter of conjecture. 
It is impossible to make any precise calculation of analysis I on ac- 
count of the decomposed condition of the hornblende and ignorance as 
to its composition. .The rock is made up of microperthite, chloritized 
hornblende, magnetite, and quartz, with much hematite stain and a little - 
_apatite. The hornblende constitutes at least 10 per cent of the rock, and 
the excess of silica which has crystallized as quartz is between 5 and 10 
per cent. 
The rock of dike 9, analysis III, is quite abnormal, none of the other 
dikes being so basic. It consists essentially of feldspar, a portion of 
which is microperthite, biotite, and apatite, with a little magnetite. 
There is a little calcite and an exceedingly slight amount of what seems 
to be augite. The main difficulty in the way of a calculation arises from 
the uncertainty regarding the composition of the biotite, which has not 
been analyzed and which is abnormally rich in iron. The appended 
calculation is based on the assumption that it is a normal biotite of the 
composition (Si0,), Al, (Mg, Fe), K, 
Calculation of Analysis III 
egal Apatite. | Biotite.| Magnetite. | Feldspar. | Residue. 
SiO see oes ST Metall eiette a OS EA aioe AW cay 656 21 
NO TSO Nae Gee GiGi Uae eS ae 11405 2a 
ISON iaro cs atsere Di pei BEN aeaah fe | eens ae RTS eee ee a 6. c 
Ia Oeeeees So tabe SO ee et ae 87 7 re EMER Pee Nc 6a oE coc 
CAO a tesa 56 Bane be arate eee sec eM EUs sa Vase 7 {2 
Mo a Wyse artes ADE = wie Vat A WIA sat eel Scola tll eee 
1A O i ragiento catered (oo ie Pehoaeeren ieee OO) Mowaee SIRS Se 0.0 
Na Os tee ee SRE ate nse lhoh x. < le Se tae 104 13 
ERO eet rake ll Ti ree oa iene 
Ce ee ores ci ei 11 (Ra Neer e oatal (Re ete emer ices Peete GMa iacsid bie. 0 
All the ferrous oxide, with the exception of the slight amount neces- 
sary to form magnetite, and all of the magnesia, were assumed to be in 
the biotite and used as the basis for its caleulation. The residue of 
alumina was then used as the datum for the feldspar calculation. The 
whole as it stands is very unsatisfactory, but the deficiency in alumina 
is likely to be explained by the fact that the assumed composition of 
