On the Serpentine Rocks of Hoboken, N. J. &c. 17 
cent. of volatile matter, and then acquires a pale brownish 
tinge. Decomposed by acids, without calcination, which 
obstructs the solution, it afforded 30 percent. of silex, about 
52 per cent. magnesia, with only two parts in the hundred 
of oxid of iron, and not a vestige of chrome, alumine or 
lime. So that as we shall perceive in the sequel of this es- 
say, the magnesian earth pervades this formation and all its 
concomitant minerals in a manner hitherto unparalleled. 
It is unnecessary for me to add any thing concerning the 
rare and singular mineral of this locality known as an al- 
most pure hydrate of magnesia, but which, though perfectly 
colorless and transparent, still conceals an essential though 
minute portion of iron, and about 30 per cent. of water. 
That part of the mineral which presents a greenish colour, 
with the lamina less flexible and diaphanous, is a contamina- 
tion, by degrees approximating it to a contiguous substance, 
which as a peculiar mineral I shall distinguish by the name 
of marmalite.* Although this hydrate presents a high de- 
gree of purity and homogenity, it has never yet been detec- 
ted in a proper crystalline form. Its lamina indeed appear 
almost always fasciculated in different directions, and insome 
specimens I have traced itinto a magnesian marble, present- 
ly to be described, in which it presents triangular lamina di- 
verging from a common centre so as to exhibit a radiating 
circle. 
The substance above mentioned, which 1 shall term 
magnesian marble, appears as it were, to form a continua- 
tion of the same veins which afford the magnesian hydrate. 
Its colour is commonly, and constantly when pure, of a per- 
fect white, sometimes of a very close grain, and with a frac- 
ture inclining to the conchoidal, but commonly splintery. 
The most compact kind even gives some sparks with steel, 
in the same fragments there are also almost always portions 
of a lamellar or sparry texture, the lamina as in marble, 
but more hyaline, crossing each other or implicated in dif- 
ferent directions. In the cavities also occur small clusters 
of limpid crystals, too minute for very accurate investigation, 
but as far as visible through the microscope, presenting 
elongated six-sided prisms, with faces of unequal breadth, 
* From papatpa, to shine, in allusion to its pearly and somewhat metal- 
ic lustre. : 
Vor. IV.....No. 1. 3 
