ANHYDRITE.—GYPSUM. 83 
used in agriculture and the arts, are very generally known under the name of gypsum, or 
plaster ; which latter name, however, is more appropriate when applied to the powdered mineral. 
When the masses of gypsum are heated, the water which is in combination is given off, and 
they are converted to a dry, white powder, like flour, which is called plaster of Paris, and has 
the important property of hardening when mixed up with a small quantity of water. The 
operation of expelling the water is called “‘ boiling," although no water is used in the opera- 
tion. The escape of the water in the form of steam keeps the mass in continual agitation. 
Sulphate of lime also occurs in nature without any water, and forms another mineral species, 
called anhydrite. This has the following composition in 100 parts: 
Lime (C, O) 41.2 
Sulphuric acid (S O,) 58.8 
100.0 
It is generally found associated with gypsum, and the two species are often found to pass into 
each other, forming a compound, with a smaller quantity of water than pertains to gypsum. 
Captain Pope observed many of these varieties of gypsum on the line of survey, and it is 
possible that beds of anhydrite were also passed. The specimens which I find in the collec- 
tion consist of the pure, transparent selenite (Nos. 7 and 25); a red, fibrous variety (No. 15); 
and a new and peculiar mass (No. 14), which is different from any hitherto described. 
The descriptions already given of selenite will suffice for Nos. 7 and 25, but No. 14 deserves 
more than a place in the catalogue. 
The specimen is about five inches long and two thick, and was taken from the banks of Dela- 
ware creek. It at first appears to be a stratified mass, being marked with regular lines like the 
divisions between strata. The main portion of the mass is white, opaque, and amorphous gyp- 
sum, but it is in combination with transparent selenite. ; 
The white amorphous portions are in thin and parallel layers, about one-tenth of an inch 
thick, and they are separated by thin sheets of a different color, and about as thick as a stout 
card. The examination of these thin sheets shows them to consist of carbonate of lime. This 
result, which was unexpected, is very curious and interesting, and the formation of the mass 
cannot be readily explained. On the weathered surfaces of the specimen these layers of carbon- 
ate of lime are more distinct than on a freshly fractured surface, appearing. at first as if they 
resisted decomposition better than the sulphate. A closer examination shows that the decom- 
position is most rapid at the line of junction between the carbonate and the gypsum, so that 
both layers stand out from the surface. : 
The transparent selenite, which traverses this mass, cuts obliquely across the opaque layers, 
both of the gypsum and the carbonate, cutting them off like a dyke traversing stratified rocks. 
Some of the layers of carbonate are, however, found in the mass of the selenite, preserving 
their parallelism and general characters, but not exactly parallel with the outer layers. The 
principal cleavage of the selenite is oblique to the layers, being inclined at an angle of 
about 45°. 
An hypothesis which will explain all these phenomena is not easily formed. The whole 
appearance of the specimen, its stratified character, suggests its origin by successive deposition ; 
but the extreme regularity in the thickness of the layers, and the absence of all granular struc- 
ture, forbid this conclusion. It may, however, have been deposited from solution in successive 
layers, and the crystalline portion may have been subsequently formed from the material so laid 
down. It appears more probable that the whole mass is due to a segregating or crystalline 
force acting while the solution of lime was diffused in the strata of earth or sand, and bringing 
the particles together in one seam, as water, during its crystallization, is sometimes separated 
in layers from a bank of earth or clay. T r «temas 
5k 
