Petrography of Shcfford Mountain. — Dresser. 209 
chemical analyses already quoted, the minerals are practically 
identical in composition while the "schiller" structure is 
merely a structural difference which may at least be of second- 
ary origin. From its chemical affinities as shown in the fol- 
lowing analyses, however, the rock is classed as nordmarkyte. 
I 
II 
III 
IV 
SiOa 
G5.43 
64.04 
64.63 
65.43 
TiOo 
.10 
.62 
Zr O2 = 
.50 
= .11 
AI2O3 
16.96 
17.92 
18.15 
16.11 
FeaOs 
1.55 
.96 
3.05 
1.15 
FeO 
1.53 
2.08 
2.85 
MnO 
.40 
.23 
1.00 
.23 
CaO 
1.36 
1.00 
1.54 
1.49 
BaO 
none 
.03 
MgO 
.22 
.59 
.50 
.40 
KoO 
5 36 
6.08 
4.79 
5.97 
NaaO 
5.95 
6.67 
5.80 
5 00 
P2O5 
.02 
.13 
CO2 
none 
trace(?) 
SO3 
.06 
F 
FeSo 
= .08 
= .07 
CI 
M ' 
.05 
H2O 
.82 
1.18 
1.08 
19 
99.86 101.37 100.54 100.18 
I. Nordmarkyte, Shefford. Analysis by Connor. 
II. Nordmarkyte from Tonsenns, Norway. Cited by RosenLusch in 
"Elemente der Gesteinslehre."' 
III. Quartz Syenyte, Dike Rock, Fourche Mt., Arkansas. Analysis by 
Brackett, Described by J. H. Williams. 
IV. Syenyte, Mt. Ascutney, Vermont. Analysis by HilleVirand, U. S. 
G. S. Bull., 148. Petrographic data by Daly and Jaggar. 
Piilaskyte. This rock is somewhat variable in appear- 
ance passing from a gray and somewhat porphyritic trachyte- 
like rock in the central part of the mass to a greenish and 
more distinctly porphyritic rock along its margin. Small crys- 
tals of black hornblende and larger ones of feldspar can often 
be seen in the feldspathic ground mass. By the aid of the 
microscope the feldspathic phenocrysts are found to consist 
of both orthoclase and plagioclase. The former appears to 
predominate in the interior, while towards the exterior of the 
mass plagioclase becomes more abundant, a variation that 
seems to be analogous to that described by Cross* in the Game 
*Geolos:v of Silver Cliff and the kosita Hills, Colorado. Whit.m.xn Ckoss, 
1 7th An. Kept., U. S. G. S., p. 30G. 
