MUSEUM OF COMPARATIVE ZOOLOGY. 241 
Occasional small structureless areas are seen which with crossed nicols 
polarize feebly or are isotropic ; one instance was noted where there was 
the faint appearance of a few minute lath-like forms in radial arrange- 
ment, as though plagioclase crystals had separated from a glassy magma. 
This mineral gives no gelatinization with hydrochloric acid. On sepa- 
rating the rock powder by gravity solutions, grains of this mineral settled 
between 2.80 and 2.60, and more came down between 2.60 and 2.51. 
Microchemical tests with hydrofluosilicic acid on these grains gave pretty 
abundant cubes of potassium and some prisms of sodium, the thorn-like 
forms of calcium were also noted. This substance would thus appear 
to be of a feldspathic nature and not nepheline since it did not 
gelatinize. This determination would be of importance were the quantity 
of the mineral large, but it perhaps does not compose more than one 
per cent of the rock mass. 
CLASSIFICATION. 
Since the study of this rock has begun, a complete chemical analysis 
has been found necessary to determine its position definitely ; until this 
has been made, only the possibilities can be given. The rock has much 
in common with the group of Fourchites of Dr. J. Francis Williams.4 
If it is regarded as belonging to this group, and is named according to 
the predominating minerals, it would be called an augite amphibole 
fourchite. The occurrence of a feldspathic constituent rich in potassium 
in the rock would tend to exclude it from this group, however, since 
these rocks properly contain a lime-soda feldspar, nepheline or leucite ; 
for this reason it may seem better, since the amount of this constituent 
is very small, to associate this rock with those of the pyroxene group 
(pyroxenite) although these have been regarded as containing no feld- 
spathic constituent. In this latter case this would be the first dike rock 
of the group. 
This dike is interesting as being the first of the group of basic dikes 
found in the Eastern United States which has a geological age determin- 
ably later than the Carboniferous ; the rocks which it intersects being 
above the middle of the Connecticut Triassic. Of course the idea that it 
is of later age than the Triassic is not excluded, though the fact that it 
is broken by a small fault might be brought forward as an argument 
for the intrusion of the rock before the time of deformation, which is 
conceived to have followed closely the Triassic deposition. The wide 
difference in character between this rock and the Triassic effusives may 
1 Arkansas Geol. Survey, Ann. Rep., 1890, Vol. II. p. 107. 
