ee e TOET OR 
gi, 
33.84 
1893.] Mineralogy and Petrography. 273 
declares that the earlier description applies only to that phase of the 
rock occurring in the northern and the southern portions of its extent. 
The pyroxene throughout the dyke is aegerine. Cancrinite and soda- 
lite are both fairly abundant in it. An analysis of specimens collected 
from about the point visited by Emerson gave: 
SiO, AlO, FeO; MnO CaO MgO K,O Na,O Loss Total 
50.36 19.34 6.94  .41 3.43 notdet. 7.17 7.64 3.51=99.80 
Eleolite porphyries with a tinguaitic groundmass are closely asso- 
ciated with the more abundant syenite, and along the eastern side of 
the great dyke are smaller ones of ouachitite and fourchite. The basic 
material of these small dykes, when first’ studied, was regarded as 
porphyrite. Contact effects produced by the intrusion of the syenite 
through the surrounding shales are noticed on the east side of the 
dyke, where the sedimentaries have been changed to biotitic hornfels. 
Mica Peridotite from Kentucky.—A mica peridotite’ from a 
dyke in Crittenden Co., Ky., is composed essentially of biotite, serpen- 
tine, and perofskite, with smaller proportions of apatite, muscovite, 
magnetite, chlorite, calcite, and other secondary products. The bio- 
tite and serpentine constitute about 75% of the entire rock. The mica 
is in large plates in which are scattered the grains and shreds of Ser- 
pentine. The composition of the rock follows: 
TiO, Al,O, Fe,0, FeO CaO MgO K,O NaO H,O P,O; 
‘3.78 588 7.04 516 946 22.96 2.04 38 7.50 .89 
and small quantities of Cr,O,, MnO, NiO, CoO, BaO and CI. The rock 
represents a new type of peridotite in which biotite takes the part of 
an amphiboloid in the more usual types. 
Rhyolites in Maryland and Penn.—G. H. Williams’ has iden- 
tified an extensive series of old volcanic rocks in the South Mountain 
region of Pennsylvania and Maryland. The rocks have hitherto been 
considered sedimentaries, but to the writer they exhibit all the pecu- 
liarities of eruptives, though some of the beds are fragmental tufas 
and breccias. The two principal types are rhyolite and basalt. The 
former possesses all the features of recent eruptives, such as flowage 
5Amer. Jour. Sci., III, xxxviii, p. 130. 
6J, S. Diller, Amer. Jour. Sci., xliv, 1892, p. 286. 
TIb., xliv, 1892, p. 482. 
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