Gypsum Deposits in Northern Manitoba. 359 
form of very minute grains of iron pyrites. The iron 
pyrites readily oxidises into a sulphate or double sulphate 
of iron which combining with the carbonate of lime give 
as products of the double decomposition, sulphate of lime or 
gypsum, and carbonate or possibly sulphate of iron. In the 
Cretaceous shales of the Duck and Riding Mountains and of 
the Plains further west, this process is clearly seen to have 
gone on. Iron pyrites is constantly present, and the shells 
of Imocerami, Ammonites, Baculites, &c., furnish an 
abundant supply of carbonate of lime. This shale is 
therefore often filled with minute, or sometimes even large 
crystals of gypsum, and side by side with them are masses 
of ironstone or impure carbonate of iron, which, after being 
formed in the above-described way, has collected in rounded 
or lenticular nodules about a shell, fragment of a crayfish, 
or other nucleus. In the case of the Paleozoic limestones, 
however, no trace is found of the carbonate or other salt of 
iron which would have resulted from the double decomposi- 
tion, and if it was ever formed in the rock, it has since 
been dissolved away by water percolating through the 
strata. 
The gypsum may, however, have been formed in a 
different way. The whole of this country has undoubtedly 
suffered very considerable erosion since Cretaceous times, 
the shales and marls of the Duck and Riding Mountains 
having almost certainly extended much further east than 
Lake St. Martin. Many of the springs that now flow from 
these shales are strongly impregnated with sulphuretted 
hydrogen, which might readily be oxidized into sulphuric 
acid, This acid acting slowly on the beds of limestone 
would alter them into sulphate of lime without disturbing 
the stratification at all. 
Of the uses of gypsum it is unnecessary to speak. In 
the Western States, where the air is dry and atmospheric 
erosion is very small, it is used as a building stone, being 
very easily worked, and sufficiently durable and strong for 
residences and all ordinary buildings. 
sy roasting, its water of crystallization is driven off and 
