350 
DESCRIPTIVE MINERALOGY 
colour, on a quartzose rock near Westport. This is the mineral which was called Cliiltonite 
by Dr. Emmons. 
Richmond County. This mineral occurs in rolled masses of greenstone. It is also found 
in the form of thin plates, on the same rock, at Picrmont in the county of Rockland. 
In Connecticut, prehnite occurs both massive and crystallized, in the trap at Farmington. 
In Massachusetts, it is found near Boston ; in Vermont, at Bellows Falls ; and in New-Jersey, 
at Paterson, Bound brook and elsewhere. 
THOMSONITE. 
[In honour of Dr. Thomas Thomson, of Glasgow.] 
Thomsonite. Phillips, Beudant, Shepard and Dana. — Orthotomer Kuphon-Spath. Mohs. — Mesotype of 
Hauy in par-t. 
Description. Colour, when pure, snow-white; also brown and yellow¬ 
ish. It occurs regularly crystallized ; also in the form of spheres, and in 
masses having a fibrous radiated structure. Primary form a right square 
prism, Fig. 344. Cleavage perfect, parallel with M. Fracture uneven. 
Lustre vitreous, inclining to pearly. From transparent to translucent. 
Hardness 5.0. Specific gravity from 2.29 to 2.37. Before the blow¬ 
pipe, it swells up like borax, and becomes opaque and snow-white, but 
does not melt. When exposed to a red heat, it becomes opaque, very 
white and shining like enamel. The edges are rounded, but it does not 
altogether lose its shape. 
This mineral was formerly confounded with several others, under the general name of 
Zeolite. Hauy divided the zeolites into two species, which he distinguished by the names 
Mesotype and Stilbite. In 1817, Fuchs and Gehlen made an accurate chemical analysis of 
a number of zeolites, and showed that the mesotype of Haiiy contains three distinct species, 
which they distinguished by the names of Natrolite, Mesolite and Scolezite. In 1820, Mr. 
Brooke, without being aware of what had been done by Fuchs and Gehlen, showed that the 
mesotype of Haiiy ought to be divided into three species, which he distinguished by the 
names of Mesotype, Needlestone and Thomsonite. The first two of these constitute the natro¬ 
lite and scolezite of Fuchs and Gehlen ; but the third is a new species, which Mr. Brooke 
first described. He showed that these minerals differ in their crystalline shape, and in their 
specific gravity; thomsonite being the heaviest, and natrolite or mesotype the lightest.* 
The stellite and wollastonite, with which this mineral has heretofore been often confounded, 
may be distinguished by their fusibility by the blowpipe. 
* See a paper on the Minerals found in the Neighborhood of Glasgow. By Thomas Thomson, M. D. &c. Bond. Edin. and 
Dub. Phil. Mag. XVII. 407. 
