REVIEWS 189 
and the destruction of great quantities of pre-Cambrian rocks. Although 
these deposits do not compare with those of the Huronian and Keewatin 
in extent, there never has been a period in the earth’s history when such 
areas of fresh igneous rock in such a condition for rapid disintegration 
were exposed to weathering processes, nor have there been more favor- 
able physiographic conditions for deposition than in Keewatin time. 
Regarding the relation of the iron-formation to different types of sedi- 
ments it may be shown that the normal relation of bog deposits to 
clastic sediments today is that here they lie on sand and there on clay. 
The origin of the copper ores of the Keweenawan is assigned chiefly 
to solutions of magmatic waters and to a lesser extent to the leaching 
of the igneous and sedimentary rocks by thermal waters. While this 
view is not entirely new it is much better established here than ever 
before by the application to the problem of rock alteration, the knowl- 
edge now possessed of the difference between the results of alteration by 
thermal solutions and by ordinary weathering processes. The presence 
of considerable chlorine in the deep-mine waters and the general scarcity 
of sulphides suggest that the chloride of copper was the principal com- 
pound in which the copper was originally transported. The copper 
was then brought to the metallic condition through the influence of 
ferrous compounds. The decreasing copper content of the deposits 
with depth is assigned to the nature of original deposition and not to 
secondary enrichment, as very little of the native copper is dissolved 
and transported. 
The silver deposits on the north shore of Lake Superior are also 
regarded as having their origin in the Keweenawan, basic, intrusive 
rocks, and it is shown that chlorine is present to a marked extent in the 
deep-mine waters. From observations of certain similarities in geo- 
logical characters it is therefore concluded that the Keweenawan copper, 
the Silver Islet silver, the Bruce Mines copper, the Sudbury nickel, and 
the Cobalt silver deposits are portions of a great metallographic province. 
In a brief account of the Paleozoic rocks on p. 615, the term Pro- 
terozoic is introduced and made to include the Archean as well as the 
four members of the Algonkian group. This is not the sense in which 
the term has been used by other geologists and we surely have enough 
geological terms with two or more meanings. ‘The author’s use is a 
pointed illustration of the unsatisfactory usage which led to the adop- 
tion of the term Proterozoic, that is the illogical practice of including 
under a term of inferior rank the four great systems, Keweenawan, 
Upper Huronian (Animikie), Middle Huronian, and Lower Huronian, 
