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-lil^e 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. GO and 2.51. 

 Microchemical tests with liydrofluosilicic acid on these grains gave pretty 

 abundant cubes of potassium and some prisms of sodium, the tliorn-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.-' 

 If it is regarded as belonging to this group, and is named according to 

 the predominating minerals, it would be called an augite ampliibole 

 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 lias 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. 



