~ 
BE ee 
[Boor II. 
84. : GEOGNOSY. 
or coarse granular amorphous aggregates. H.3:5—45. Gr. 3°7—3°9. 
Composition—ferrous carbonate or carbonate of the protoxide of iron ~ 



ve 
; 
[Se 
a 
x) 
(=ferrous oxide 62°07, carbon dioxide 37°93), but seldom with the 
theoretically pure composition ; usually with an intermixture of other 
carbonates (especially of manganese, magnesium, and calcium), and _ 
in the coarse varieties with clay and many other impurities. Occurs — 
erystallized in association with metallic ores, also in beds and veins 
of many crystalline rocks, particularly with limestones ; the compact 
argillaceous varieties (clay ironstone) are found in abundant nodules — 
and beds in the shales of Carboniferous and other formations where 
they have been deposited from solution in water in presence of 
decaying organic matter (see pp. 115, 175). 
SutpHates. Among the sulphates of the mineral kingdom, only | 
three deserve notice here as important compounds in the constitution 
of rocks—yiz., calcium sulphate or sulphate of lime in its two forms, 
Anhydrite and Gypsum, and barium sulphate or sulphate of baryta 
in Barytes. 
Anhydrite. Orthorhombic; fibrous, lamellar, granular. H. 3—3°5. 
Gr. 2:°8—3. Composition—anhydrous calcium sulphate (= sulphuric 
acid 58°82, lime 41:18). Occurs more especially in association with 
beds of gypsum and rock-salt. 
Gypsum. Monoclinic; granular, foliated, fibrous, massive. H. 
1:5—2. Gr. 2°2—2°4. Composition—hydrous calcium sulphate (= 
sulphuric acid 46°51, lime 32°54, water 20°95). Abundant as an 
original aqueous deposit in many sedimentary formations. (See p. 115.) 
Barytes (Heavy Spar). Orthorhombic; also crested, fibrous, — 
coarsely laminated. H.3—3°5. Gr.438—4:7. Composition—barium 
sulphate (=baryta 65:7, sulphuric acid 34:3). Frequent in veins 
traversing rocks of many different kinds, and especially associated 
with metallic ores as one of their characteristic vein-stones. 
PHospHATES. ‘The phosphates which occur most conspicuously 
as constituents or accessory ingredients of rocks are the tricalcic 
phosphate or Apatite, and triferrous phosphate or Vivianite. 
Apatite. Hexagonal in six-sided prisms; colourless, grey, green, 
yellow, and red, usually opaque except in minute crystals; also 
massive (phosphorite) H. 5. Gr. 8:16—3:22. Composition— 
Neutral phosphate of calcium, with fluoride or chloride of calcium, 
or both. Occurs in many igneous rocks (granites, basalts, &e.), in 
minute non-pleochroic needles giving faint polarization tints ; also as 
massive beds associated with metamorphic rocks. 
Vivianite (Blue iron-earth). Monoclinic, also often globular and | 
earthy, _H. b5—2. Gr, 26—2:7. Usually bluish or bluish-green. 
Composition—hydrous triferrous phosphate (= protoxide of iron 
43°03, phosphoric acid 28:29, water 27-95, but the iron frequently more 
or less altered into peroxide). Occurs crystallized in metalliferous — 
veins; the earthy variety is not infrequent in peat-mosses where 
animal matter has decayed, and is ‘sometimes to be observed coating 
fossil fishes as a fine layer like the bloom of a plum. 
Fnuormens, The element fluorine, though widely diffused in 

