186 REVIEWS 
In the classification of the Keweenawan igneous rocks, p. 396, by 
A. N. Winchell, the term plagioclasite is used as a synonym for anortho- 
site. While this term is not so euphonious as anorthosite and might 
include rocks of greater range in chemical composition than that usually 
designated by this term it has simplicity and the advantage of corre- 
sponding in use to the terms amphibolite, pyroxenite, and leucitite. 
The Lake Superior sandstone is for various reasons assigned to the 
Cambrian. 
Beginning with chap. xvii the iron ores are discussed in great detail. 
The deposits are divided, chiefly on a genetic basis, into the pre-Cambrian 
sedimentary types, the source of nearly all the ore; titaniferous magne- 
tites constituting magmatic segregations in gabbros; magnetic. ores 
representing pegmatite intrusions in basic rocks; residual bog ores of 
Paleozoic age; and hematites from the Clinton series. The average 
iron content of all the original phases of the pre-Cambrian iron-bearing 
formations, not including the slates, is 24.8 per cent and the average for 
the ferruginous schists and jaspers is 26.33 per cent. If the average be 
taken for the formations including the ores the content of iron is 38 per 
cent showing the influence of concentration. 
About $21,600,000 has been spent in diamond drilling in the region 
at an average cost of about $3. per foot. The average cost of mining 
underground is probably about $1. per ton, and the cost of transport- 
ing ore to the furnaces (in 1907) about $2.14 per ton. The stripping 
operations in the Mesabi district annually exceed in extent those on the 
Panama Canal. 
The estimated reserves of pre-Cambrian ores is placed at 
1,905,000,000 long tons. : 
The genesis of the sedimentary iron formations is the topic of great- 
est interest in this monograph and it is interesting to see the change of 
view by the senior author. He has to a large degree forsaken his earlier 
views which held that the iron deposits were principally the products 
of rock weathering for those which consider these deposits as coming 
chiefly either directly or indirectly from igneous rocks without the 
action of ordinary weathering agencies. The fact that anyone so familiar 
with these deposits as President Van Hise should adopt the new theory 
is a strong point in its favor. ‘The hypothesis now accepted by the 
authors for the origin of the deposits is, briefly, that they are principally 
chemical sediments, that the original minerals were largely iron car- 
bonate and silicate with some ferric oxides. The greater portion of the 
ore owes its present condition to secondary enrichment under special 
