350 



DESCRIPTIVE MINERALOGY. 



colour, on a quartzose rock near Westport. This is the mineral which was called Chiltonite 

 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 Piermont 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.] 



Thomsonitc. Phillips, Beudant, Shepard and Darui. — Orthotomer Kuphon-Spath. Mohs. — Mesotype of 



Haay in part. 



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. Haiiy 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 Thomsonitc. 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 ; thomsonitc 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 foimd in Ihe Neighborhood of Glasgow. By Thomas Thomson, M. D. &c. Land. Edin. md 

 Dub. Phil. Mag. XVII. 407. 



