240 



THE GEOLOGIST. 



Pargasite, and basaltic hornblende. General composition about 

 ivTg (15), Ca (12), 11 (10), Fe (7—10), very variable. Uralite is also an 

 aluminous variety. The non-aluminous varieties are Tremolite and 

 J c^moZi^e (magnesia-lime hornblende) with (24), Ca (14), and ^q, 

 amounting to (4) in Actinolite ; and AntophylUte (magnesia-iron 

 variety) with Mg (24), ^e(14), IVIn (2). The fibrous variety. Asbestos 

 or Amianthus, has generally the composition of Tremolite or 

 Actinolite. 



In this mineral the ll is supposed to act partly as an acid, re- 

 placing the Si and forming aluminates, and partly as a base ; but the 

 subject is very obscure. It is less subject to decomposition than 

 Augite, from its containing less Ca ; but yet it is liable to many 

 series of alterations. 

 62. Hypersthene [.5]. A silicate of Mg (12—25), Fe (25—12), Ca (3), and 

 often Al and IVIn. Marked by its small contents of Ca, and wide 

 replacement by each other of Mg and Fe.' 



53. Bronzite [.5]. A siHcate of Mg (30) and I'e (9), with often some 



Mn and H, and sometimes Ca. 



54. Diallage [.5]. A sihcate of % (17), Ca (16), (7), Al (3), 



with a (2). 



55. Serpentti^e [.75]. Many varieties. A silicate of Mg (40) and J^e 



(2-6), with H (12). 



56. Talc ; Steatite, Speckstein [.428 or .437]. Average composition of 

 siHcate of Mg (32), J e (2), with fi[ (3). Steatite is the compact variety, 

 but has always a crystalline structure. Meerschaum is the amorphous 

 variety, with four per cent, additional water. 



67. Olivine, Chrysolite, Peridot [1.0]. A silicate of Mg (45) and Fe (15), 

 with very little Mn and Fe. In the green transparent variety, 

 Chrysolite, there is the highest proportion of Mg (50) and a com- 

 parative decrease of Fe, 



