32 
GYPSUM. 
hundred miles distant from Fort Belknap, and probably have no connexion with the deposit at 
that place. It appears to me most probable that Dr. Shumard’s opinion is entirely correct, and 
that the coal of Fort Belknap is of the Carboniferous era. The occurrence of the Carboniferous 
limestone in the vicinity, as shown by the Productus already mentioned, renders the Carbonif¬ 
erous age of the coal nearly certain. 1 It is most probable that this coal deposit is of great 
extent, underlying a broad area in Texas, and extending into Hew Mexico, under the great 
Llano Estacado , and possibly outcropping on the eastern flanks of the Hueco and Organ 
mountains. 
III. GYPSUM. 
General description of the mineral.—Its composition and uses.—Anhydrite.—Varieties of Gypsum in the collection.— 
Description of a specimen containing carbonate of lime.—Gypsum of Plaister Cove, Nova Scotia.—Applications of gyp¬ 
sum in the arts.—Quantity mined and sent to the United States.-—Remarks on the origin and formation of the beds. 
The geological peculiarities and relations of the Gypsum deposits which were passed during 
the survey, have already been described in the preceding portions of this report. The great 
extent of the deposit, however, and the wide distribution of the material, demand more than 
a consideration of its geological position and characters, as compared with other deposits. The 
peculiar and varied forms under which the gypsum appears, the phenomena of its formation, 
and its economical value, should be considered. As this Report will fall into the hands of many 
who are not familiar with the composition and properties of the mineral, I present a partial 
mineralogical description of the species and its varieties. 
The mineral species Gypsum consists of sulphuric acid, lime, and water, combined, when pure, 
in the following proportions: 
Sulphuric acid (S 0 3 ) 
Lime (C a O)... 
Water (HO). 
100.00 
46.51 
32.56 
20.93 
Its formula is C a O, S 0 3 -|- 2 H O. It crystallizes in beautiful transparent crystals, which 
belong to the monoclinic system, and are found with various modifications. These crystals 
may be split (cleaved) very readily in one direction, into beautiful and perfectly transparent 
thin plates, like mica, but not elastic, which are sometimes used instead of glass. These 
crystals are very soft, and may be scratched by the finger-nail, being a little softer than rock 
salt, or 1.5—2 on the scale of hardness adopted by Mohs. The specific gravity (of crystals) is 
about 2.3, being less than the gravity of carbonate of lime or limestone. It occurs of various 
colors, generally white, but sometimes grey, flesh-red, honey-yellow, ochre-yellow, and blue; 
while impure varieties are of various shades of black, brown, and red. It dissolves, without 
effervescence, in chlorohydric acid, and is soluble in 500 parts of pure water. 
It appears in nature under many different aspects, as will be seen from the number of colors 
given above. It has also many varieties of form, being found in the small and perfect crystals; 
in broad thin plates, like glass; in fibrous masses, with a peculiar silky lustre; and in granular 
masses of different degrees of fineness of grain, both translucent and opaque. These varieties 
have received different names; the transparent plates are called selenite, from selene, the moon; 
the fibrous masses are called satin-spar; and the white and fine-grained amorphous varieties, 
suitable for carving, are known as alabaster. The ordinary compact opaque masses, which are 
1 Since the above was written, I have received a collection of fossils from the vicinity of Fort Belknap, made by Dr. 
Shumard, when with Captain Marcy, on his exploration of the country of the Witchita Indians. These fossils consist of 
two or three species of Productus and Terebratula; there are also many Encrinites and Cyathophyllce, and a Pleurotomaria, from 
the coal-bed. They were collected at Belknap, and nine miles below, and at a point forty miles west. I have therefore 
marked the Carboniferous limestone on the map at these places. 
