70 CONTRIBUTIONS TO MINERALOGY. [b.ull. 262. 
These figures at once suggest a probable admixture of brucite with' 
the serpentinous material, and a microscopic examination by Mr. fJ. S. 
Diller tends to confirm this supposition. 
According to Mr. Diller the specimen is mainly composed of three 
minerals, a, b, and c. The first two have nearly equal indices of re- 
fraction, but differ widely in birefringence. The mineral a is the 
most abundant. In transmitted light it is colorless, but between 
crossed nicols it exhibits brilliant colors. The mineral b is pale greenj 
and intermingled with a. Its birefringence yields weak colors, quite 
characteristic of chlorite. In quantity it is less than one-fourth of d 
Mineral c is granular, in scattered grains and irregular groups with 
a high index of refraction and a birefringence which suggests :t car- 
bonate, but it does not effervesce with a dilute acid. It amounts to 
not more than 5 per cent of the whole. Mineral b is certainly chlorite, 
and a may be brucite or possibly serpentine. 
By applying Air. Diller's observations to the analysis of the rock, 
the proximate composition of the latter may be deduced; although 
certain assumptions must be made. The carbonate present is proba-" 
bly hydromagnesite, for that species is a frequent associate of brucite. 
The composition of the chlorite is unknown, but it may be interpreted 
as essentially clinochlore, and as proportional to the ferric oxide and 
alumina. So much assumed, the analysis gives the following approxi- 
mate results expressing the composition of the specimen: 
Hydromagnesite < r > 
Chlorite 14 
Serpentine 20 
Brucite _ 60 
E x t raneous water 1 . 
Total 100 
In this, four minerals appear instead of three, but the microscopic 
examination did not attempt to discriminate between the brucite and ; 
the serpentine. 
In order to obtain evidence confirmatory of the foregoing con- 
clusions, a few experiments were made, tending toward fractional; 
determinations. Upon digesting the powdered rock for two hours 
with cold, dilute nitric acid (10 per cent by volume), 1.32 per cent oi 
Fe 2 3 +Al 2 3 and 1:7.29 per cent Mg() went into solution. In a similar- 
experiment with cold, 20 per cent acetic acid, 0.69 Fe.,() ;! + ALO., anc 
45.64 MgO were extracted. Brucite dissolves readily in acids of th( 
indicated strength, but some chlorite was evidently attacked as well 
I also found that ordinary serpentine was quite appreciably acted upoi 
by weak acetic acid. These experiments, then, merely show that th< 
rock contains a large amount of magnesium in a very easily solubl< 
