138 
ECONOMIC GEOLOGY 
pp. 84-5), the tricalcic silicate and aluminate of alite are 
decomposed by water and form an especially active lime 
hydrate (CaH,0,), which crystallizes and acts as cement, 
The tricalcic aluminate decomposes first and its calcic hydrate 
sets first. The tricalcic silicate decomposes more slowly, 
and its silica is deposited in the setting mass and hardens it, 
The celite is almost inert, so that it usually takes no part 
in the reaction but is enclosed in the set alite. 
Price—The manufacture of Portland cement on a great 
scale began in England and it was not until about 18g3 that 
it was largely made in the United States, which has since 
become the greatest producer with about 26 million tons 
per annum; Germany is second with 7 million tons; the 
British Empire makes 5 million tons; France and Belgium 
2 million tons apiece. The average price from 1870 to 1880 
was 12s. a barrel (380 lb. before 1921, later 376 1b.) ; it had 
fallen to 8s. a barrel by 1893; American competition and 
improved methods of manufacture lowered the price to 
3s. 6d. in 1908; it rose at the end of the War to 8s., and in 
[026 to 10s. a barrel, 
Gypsum CEMENTS—PLASTER OF Paris—Plaster of Paris 
is a cement made from gypsum (CaSO, 2H,0), which when 
pure consists of 79-1 per cent. of sulphate of lime and 20-9 
per cent. of water. Alabaster is the white fine-grained 
variety used for statuary : selenite is the water-clear crvstal- 
line variety. 
Primary gypsum, due to the evaporation of sea-water, 
often occurs as beds or lenticles interbedded in red shales 
or marls. Gypsum, formed by the hydration of anhydrite 
(CaSO,), has disturbed and slicken-sided margins owing to 
its expansion. 
Gypsum occurs in veins, in irregular pockets traversing 
calcareous rocks, as a limestone-gypsum breccia, and in 
masses in contact with corroded surfaces of limestones, which, 
to use J. V. Harrison's phrase, look worm-eaten. Such 
secondary gypsum is due to the decomposition of pyrites 
having produced sulphuric acid, which has invaded limestone 
and altered the carbonate into sulphate. The process happens 
on a small scale in clay and forms selenite in crystals, nodules, 
and casts of shells. This explanation was adopted by Dana 
(1871, Manual of Geology, P. 248) for gypsum deposits in