A POPULAR EXPOSITION, ETC. 437 
A POPULAR EXPOSITION OF THE MINERALS AND 
GEOLOGY OF CANADA. 
BY E. J. CHAPMAN, Ph.D. 
PROFESSOR OF MINERALOGY ‘AND GEOLOGY IN UNIVERSITY COLLEGE, TORONTO. 
(Continued from page 219.) 
The Upper Silurian Series:—This subdivision in Canada—as 
separated from the Middle Silurian. series—contains but two groups of 
strata: the Onondaga formation at the base of the series, and the suc- 
ceeding Lower Helderberg division; but the latter, as regards the 
greater part of the Province, is but feebly represented. 
The Onondaga Formation :—This division, more commonly known 
as the ‘‘ Onondaga Salt, or Gypsiferous, Group,” derives its name 
from the village of Onondaga, near Syracuse, in the State of New 
York. The abundant brine-wells of that locality belong to the group. 
In Canada, the Onchndaga deposits average in thickness between two 
and three hundred feet, and consist essentially of thin-bedded dolo- 
mites, usually of a yellowish colour, with greenish shales (chiefly 
argillaceous dolomites), and some associated masses of gypsum. The 
latter substance, so largely employed as a mineral manure, and in the 
preparation of Plaster of Paris (see Parr II1.), does not occur in 
regular beds, but in large lenticular masses, as exhibited in the 
annexed figure. The delomitic layers above the gypsum, are gener- 
rally arched, and more or less fissured ; 
whilst those on which the gypsum 
rests, retain their normal condition. 
The disturbance, consequently, by 
which the upper beds have been af- 
fected, was evidently produced by some . Fig. 226 
after cause connected with the presence of the gypsum. The peculi- 
arity was originally explained by assuming the gypsum to have been 
derived from the surrounding rocks by the gradual action, upon these, 
of springs containing a certain amount of free sulphuric acid : springs 
of this kind occurring, at present, at several localities in Western 
Canada and New York. But it is now regarded l:y Professor Sterry 
