328 On the Geology of the Northwestern Regions of America. 
ion with extremely copious and rich salt-springs. Where they 
approach the crystalline rocks, they are found, like those of Lake 
Winipeg to be highly magnesian,—a circumstance which ma 
deserve attention with reference to the hypothesis of dolomization, 
which regards the introduction or development of magnesia as 
subsequent to the deposition of the calcareous matter, and as 
connected with the proximity of masses containing that earth and 
heated to a very high temperature. Among the fossils collected 
from this district which are in the British Museum are Spirifer 
erispus, Dalm.?, Rhynchonella phoca, Salter, Atrypa levis, Va- 
nuxem, Atrypa reticularis, an Orthis, two small Spirifers, like S. 
itrapezoidalis, Dalm. and S. pisum, Sow., and fragments of an 
Encrinital stem like that of Actinocrinus. Sir George Back, on 
his expedition down the Great Fish River, collected some frag- 
ments of Corals along the south shore of Slave Lake, which 
were considered by Mr. Stokes and by Mr. Lonsdale to belong 
some to Catenipora escharoides, and one to the genus Siromato- 
pora of Goldfuss, and probably to his species 8. polymorpha. 
From the circumstance of these fossils being chiefly Upper Silu- 
rian, it has been conjectured with every appearance of proba- 
bility, that the salt-springs may belong to the “Onondaga Salt 
Group” of [below] the “Helderberg division” of the New York 
system. 
Carboniferous Series (Mountain Limestone ?).—Some of the 
organic remains procured by Sir John Richardson on a previous 
expedition from other points along the Mackenzie River would 
appear to indicate an ascending order in some of the deposits of 
that district from the Devonian limestones and the shales con- 
recent age. In some specimens from the limestone* of the 
ments with which this part of the country is covered. 
* The limestone of the “Ramparts,” which a again lower down at a spot 
called the “Narrows,” is continued in a eye irection to the Rocky Mountains, 
the lower elevations of which are composed of it im that portion of the range through 
which Peel’s River takes its course. It has all the characters of the Mountain 
Limestone of English Geologists,—a formation extensively developed in pee 
America, where, as will be subsequently noticed, it has been clearly identified by 
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