208 
The A 
merican 
Geo 
dog 
ist. 
October, 1901. 
I 
II 
Ill 
IV 
SiOo 
65.15 
65.90 
65.66 
67.06 
AI2O3 
20.55 
19.46 
20.05 
19.00 
FesOs 
.44 
trace 
FeO 
trace 
Mn 
.13 
CaO 
.73 
.2S 
.67 
MgO 
.18 
K2O 
6.39 
6.55 
6 98 
6 93 
NaaO 
6.67 
6.14 
6.56 
7.00 
H2O 
.50 
.12 
2.19 
99.90 98 90 100.04 99.99 
Augite in this rock commonly occurs in stout columnar 
crystals and in ordinary light is either colorless or slightly 
greenish, without perceptible pleochroism. It is frequently 
associated with smaller grains of magnetite and both may en- 
close needles of apatite. Hornblende in the thin section is 
green or yellow in ordinary light, never brown as in essexyte. 
Sections parallel to c = deep green ; b -= yellowish green ; 
a = straw-color. 26° was the greatest value found for c /\ c. 
Biotite polarizes in unusually brilliant tints, probably in- 
dicative of a larger proportion of iron than usual in its com- 
position. Sphene is frequently quite an abundant accessory 
and occasionally a few grains of quartz are seen. 
In structure the rock is coarsely granitic. The absence of 
quartz in sufficient amount to form a cementing material for 
the other constituents tends to render it friable on exposure 
to the atmosphere. It is easily disintegrated and, where pro- 
tected from heavier glaciation, is often reduced to a loose mass 
of rectangular grains of feldspar for several feet in depth. 
It will be seen from the above description that this rock 
bears a close resemblance to some of the syenyte types of 
southern Norway which have been made classic by the work 
of Prof. \\'. C. Brogger. Compared with thin sections of 
laurvikyte from Frederiksvarn, or in other cases with nord- 
markyte from Christiana, the only discernible differences are 
that feldspar from Shefford is microperthite, rather than 
kryptoperthite, and that a "schiller" structure frequent in the 
augite of the laurvikyte is not seen in that from Shefford. The 
first mentioned difference is only one of the texture of the al- 
bite-orthoclase intergrovvth, for, as has been shown by the 
