ALUMINA. 351 



Composition. Silica 38.30, alumina 30.20, lime 13.54, soda 4.53, water 13.10 {Ber- 

 zelius). 



Geological Situation. In this country, as elsewhere, thomsonite has been found only in 

 trappean rocks. 



localities. 



Rockland County. In the rifts of greenstone at Piermont, there is a mineral which, in its 

 external characters, closely resembles some varieties of thomsonite ; and the latter mineral 

 has been unhesitatingly credited to the similar formation at Bergen hill, New-Jersey. 



I will here only remark, that all the specimens which I have examined are clearly refer- 

 able to another species, the Stellite of Thomson. But I do not wish to be understood as 

 asserting that thomsonite has not been found at the localities in question. The above descrip- 

 tion has been introduced, and the doubts expressed, for the purpose of directing the attention 

 of our mineralogists to the subject. 



LAUMONITE. 



[From its discoverer, M. Gilbert Laumont.] 



Laumonite. Hatiy, Cleaveland, Phillips, Beudant and Shepard. — Lomonite. Thximson. — Diatomous Zeolite, 

 or Laumonite. Jameson. — Diatomer Kuphon-Spath. Mohs. 



Fig. 345. Description. Colour white, passing into reddish, yellowish or greyish 



tints. It occurs regularly crystallized, and in distinct granular concretions. 

 The primary form is an oblique prism, of which the inchnation of the ter- 

 minal plane is from one acute angle to the other. Fig. 345. M on M' 86° 

 15' ; M on P 113° 30'. Cleavage parallel to its lateral planes and both dia- 

 gonals. Lustre vitreous, inclining to pearly upon the more distinct faces 

 of cleavage. When fresh, it is transparent ; but on exposure to the atmo- 

 sphere, it very soon becomes opaque, and so soft as to yield to the pressure 

 of the finger. Very easily frangible. Hardness 3 .50, determined with diffi- 

 culty ( Thomson). Specific gravity from 2 . 20 to 2 . 30. Before the blow- 

 pipe, it loses water and melts into a spumous mass. It gelatinizes in acids, and becomes 

 electric by friction, if insulated. 



Composition. A specimen from the Isle of Skye — Silica 52.04, alumina 21.14, lime 

 10,62, water 14.92 (ConneZ). 



LOCALITIES. 



Putnam County. This mineral has been found in crystals of the primary form, associated 

 with stilbite in feldspar in Phillipstown. The locality is believed to be exhausted. In Con- 

 necticut and Massachusetts, laumonite has been found in small quantities in amygdaloid. In 

 Pennsylvania, it has been met with in fine crystals in Chester county. 



