162 A POPULAR EXPOSITION OF THE 
_ transparent cleavable varieties are often called ‘selenite,’ and the 
fibrous and fine granular varieties are known by lapidaries as “ satin 
spar,” and “alabaster,’’—-names, however, sometimes applied to varie- 
ties of calc spar. Gypsum, when deprived of its water by a low heat, 
forms the well known plaster of Paris. In Western Canada, this 
most useful mineral occurs abundantly in the Gypsiferous or Onon- 
daga Salt Group of the Upper Silurian Series (see Part V. of this 
Essay): as in the townships of Dumfries, Brantford, Oneida, Seneca, 
and Cayuga, more especially, along the valley of the Grand River. 
The gypsum does not occur in beds, properly so-called, but in vast 
irregular masses, supposed by Mr. Sterry Hunt, (Comptes Rendus, 
1855, and Hsquisse géologique du Canada,) to arise from the action, 
on the surrounding limestone strata, of springs containing free sul- 
phuric acid. In these localities the gypsum is more or less mixed 
with carbonate of lime. Fibrous and other varieties occur also in 
the vicinity of Owen Sound, and throughout the tract of country, 
generally, between the eastern extremity of Lake Erie and the mouth 
of the Saugeen. Likewise, here and there, in small cavities and 
fissures in the Niagara limestone and older rocks. 
APPENDIX. 
A Classified List of the Canadian Minerals described above. 
In this list, which is intended to serve as a kind of Index to the 
minerals described in the present Part of our Essay, each substance 
will be found arranged under the chemical sub-division to which it 
belongs. The letters and numerals within brackets, refer to the 
groups and sub-groups of the Arrangement adopted above. 
1. Simple Substances. 
Native Gold, (B. 1.) Native Platinum and Osmium-Iridium, 
(B.i.) Native Silver, (B. 1.) Native Copper, (B. 1.) Graphite, 
(B. 2.) , 
2. Arsenides and Sulphides, (Combinations of arsenic, or sulphur, 
with metallic bases.) 
Arsenical Nickel, (4. 2.) Sulphide of Silver, (B. 1.) Galena or 
Sulphide of lead, (B. 3.) Sulphide of Copper, (B. 3.) Purple 
Copper Pyrites, (B. 3.) Copper Pyrites, (B. 3.) Zinc Blende, 
