On the Serpentine Rocks of Hoboken, N. J. &e. 17 
cent. of volatile — 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 
-Yare and singular mineral of this Joeality known as an al- 
most pure hydrate of magnesia, but which, though perfectly 
colorless and transparent, still conceals an essentia h 
minute portion of iron, and about 30 per cent. of water. 
That pees of. the mineral see presents a greenish colour, 
as guish by the nai 
of marmalite.* Although this hydrate | rieesehing a Bich vad 
gree of purity and homogenity, it has never yet been detec- 
d in a proper crystalline form, — Its lamina indced appear 
almost always fasciculated in different directions, and in some 
‘specimens I have traced itinto a magnesian marble, present- 
ly to be nie in which it presents triangular lamina di- 
© common centre so as to y opabe a etd 
"The sabstihiees above ‘otal: : 
magnesian marble, appears as it were, to form a 
tion of the same waits which afford the magnesian 
{ts colour is commonly, and constantly when pure, of a p 
fect white, sometimes of a very close grain, and with a ac- 
ture inclining to the conchoidal, but commonly splintery. 
The most compact kind even gives some s with steel, 
in the same fragrnents there are also almost ect portions 
aes lamellar or sparry tex sextant, er letnina ae: mgeble, 
a : more hyaline, ee Atos i: 
lirections. In the cavities also cet sl ters 
impid crystals, to 
bak as far as visible through the microscope, presenting 
sloogeet a x- + stomps wayhnenel a breadth, 
