T. S. Hunt on Ophiolites. 225 
gentlemen, I have been furnished with a series of specimens, 
which have permitted me to make a.careful examination of the 
Roxbury ophiolite, : 
- _ Some portions of the rock appear as a mottled granular mass, 
having a hardness of about 4-0, with an uneven fracture, and pre- 
_Senting cleavable grains of magnesite; the colors vary from 
blackish-green to greenish-white, and the rock is susceptible of a 
high polish. Other specimens are white and crystalline, with 
humerous greenish-grey bands, the whole arranged in parallel 
yers, as if stratified, and resembling closely some varieties of 
gneiss. The rock cleaves with these layers, which contain ser- 
pentine and tale, intermingled with carbonate of magnesia. This 
mineral, as described by Drs. Jackson and Hayes, is nearly pure 
in the white portions, and has a hardness of 4’0, and a density 
of 2°99—3-00, according to my determinations. Dr. Hayes 
found for its composition, carbonic acid 48°80, magnesia 45°60, 
et a little silica 8°60, silicate of protoxyd of iron 1°96 
This result corresponds clos@y with my own. I obtained from 
100 parts, 2-76 of tale, and 1:82 of silica, besides 2°40 of per- 
oxyd of iron, corresponding to 8°48 of carbonate of iron, the 
nitrate of ammonia; but there is also present a portion of silicate 
‘ron and magnesia, decomposed by acids. In my analysis the 
peered magnesite was digested fora long time at a boiling 
fat with hydrochloric acid; the insoluble portion was then 
boiled with strong sulphuric acid, and from the residue the 
ita Was removed by a solution of carbonate of soda, the tale 
ning. # 
he tale thus purified from magnesite and serpentine by suc- 
edi treatments with hydrochloric and sulphuric acids and car- 
eag of soda, was gently ignited, and then decomposed by 
on with carbonate of soda; it gave:— 
ilica, e 3 2 62°60 
sia, . * 2 ‘ - - 31:30 
Alumina and i . 3 é - - 406 
rT and ribs . zs eB , P : 204 
ei 100-00 
bon; the analysis of Dr. Hayes just cited, the 48-80 parts of car- 
acid are sufficient only for 44°36 parts of magnesia, 
8 Silicate, 9 dark i f the rock was pulverized 
. ’ ark-green portion of the Tut , . 
boiled for a long time with dilute nitric acid, whi dissolve 
SECOND SERIES, VOL, XXV, NO. 74.—MARCH., 1858. 
29 
