Petrography of Shcfford Mountain. — Dresser. 207 
(cited by Rosenbusch, loc. cit.); (IV) The original Essexyte, Salem 
II III IV 
50.47 50.50 47.94 
.51 1.91 .20 
18.73 17.64 17.44 
4.19 5.41 6.84 
4.92 4.02 6.51 
Neck, Mass. 
I 
SiOs 
53.15 
TiOs 
1.52 
AI2 O3 
17.64 
FeaOs 
3.10 
FeO 
4.65 
MnO 
.46 
CaO 
566 
BaO 
.13 
MgO 
2.94 
K2O 
3.10 
NaaO 
5.00 
P2O5 
.65 
CO2 
.•39 
SO3 
.28 
CI 
.07 
HaO 
1.10 
.11 
7.91 7.47 
3.48 3.33 2.02 
3.56 3.02 2.79 
4.62 5.52 5.63 
.10 .92 1.04 
.58 .45 2.04 
99.84 100.09 100.63 99.92 
Nordiiiarkytc. This rock was described tinder the head 
of granitoid trachyte by Logan (Geology of Canada, 1863, P- 
653) as "being made up in a great part of a crystalline feld- 
spar, with small portions of brownish black mica, or black 
hornblende, which are sometimes associated. The proportion 
of these minerals is never above a few hundredths, and is often 
less than one-hundredth. The other mineral species are small 
brilliant crystals of yellowish sphene, and others of magnetic 
iron, amounting together probablv to one-thousandth of the 
mass." 
To the essential minerals there should be added a nearly 
colorless augite, which commonly equals the hornblende in 
amount and is a rather more persistent constituent of the rock. 
The feldspar is almost wholly a microperthite, which as the 
following analysis by Hunt (Geology of Canada, 1863, p. 
476) shows, answers approximately to an isomorphous mix- 
ture of albite and orthoclase molecules in the proportion 
of 3 : 2. 
I. Feldspar, Shefford. Analysis by Hunt. 
II. Kryptoporthite, Laurvik, Norway. Analysis by Emelin, Syenit- 
pigmatitgange, p. 524. 
III. Anortlioclase, Marblehead Neck, Mass. Analysis by Clietard, 
Bull. Mus. Comp. Zool., XVI. 
IV. .Approximate theoretical composition of feldspar having the form- 
ula Aba Oro. 
