70 MINERALOGY AND LITHOLOGY. 



46. Chrysolite (Olivine) [(Mg, Fe)^ Si O4]. 



This mineral is a prominent ingredient of some of our igneous rocks. 

 It was first noticed as an ingredient of the trap at Campton falls by 

 Prof. O. P. Hubbard,* who also found it forming masses of some size 

 in a very coarsely crystalline diabase, which is found in boulders (the 

 original locality for which is not known) at Thetford, Vt. It was ident- 

 ified in the gabbro from Watervillef by Mr. E. S. Dana. When visible to 

 the eye in the rocks it has a vitreous lustre, and a greenish yellow color. 

 The easy decomposition to which it is subject makes it conspicuous; 

 and rocks like our gabbros, which, when broken, show the clearest and 

 freshest grains of chrysolite on a fresh fracture, are externally covered 

 with iron-stained pits, from which the chrysolite has rotted away. It has 

 been found, also, in granite boulders near the Crawford house. 



The following is an analysis of the chrysolite from the Waterville 

 rock, by Mr. Dana. 



Silica, 38.85 



Alumina trace. 



Iron protoxide, 28.07 



Manganese protoxide, 1.24 



Lime, 1.43 



Magnesia, 30.62 



100.21 



The analysis shows this to be the variety of chrysolite which com- 

 monly occurs in rocks, and which is usually called olivine. It contains 

 an unusually large amount of iron, — as remarked by Mr. Dana, the pro- 

 portion between the iron and magnesia being as 1:2. Hence the 

 formula for this olivine is 



( 2 Mg^ Si O, ) 



\ Fe^ Si O, i 



When present in sections of rocks prepared for microscopic study, 

 chrysolite is easily recognized. When crystalized, its sections are either 

 six- or eight-sided, the figures being the sections of the most common 

 form of the crystals of this species. This form is made by a combina- 

 tion of two prisms and a macro- and brachydome. In the rocks that I 

 have examined, the sections at right angles to the vertical axis appear 



* Ant. yeur. Science, \, vol. xxxiv, p. no. f Id., iii, vol. iii, p. 48. 



