REVIEWS 185 
Much new data relating to the iron ores has been collected, especially 
regarding their chemical composition, porosity, and moisture content, 
and the results have been depicted graphically on the triangular diagram 
which is so frequently employed in this work. It is shown that the 
phosphorous in the ore of the Mesabi district has come chiefly from the 
overlying Cretaceous rocks while in the Penokee-Gogebic district the 
phosphorous content of the ore increases with the percentage of iron 
indicating that this element was originally in the ore. In the vicinity 
of dikes the phosphorous content is high and leached dikes show a loss of 
this element so the igneous rocks appear to be the original source of it. 
No definite phosphorous minerals have been found, but the association 
of calcium and phosphorous in some of the deposits suggests the presence 
of apatite. 
The average analysis of ore from the Mesabi district for three years 
shows a decrease from 60.70 per cent to 58.83 per cent in iron and a 
corresponding increase in phosphorous. 
In the description of the Marquette district, peridotite as well as 
syenite is considered of Laurentian age and this fact is of interest since 
we have become so accustomed to thinking of this series as acid in com- 
position. Regarding the placing of slates under the Ely Greenstone, as 
is done on p. 119, the question might be raised, whether it would not be 
better to reserve the term greenstone for lithological characters and apply 
it only to chemically and mineralogically altered, basic, igneous rocks. 
The Keweenawan series is regarded as largely terrestrial in origin 
and a number of excellent reasons are given for drawing this conclusion. 
A few of these are: the thickness of the sediments and the frequent 
repetition of conglomerate beds; the feldspathic, poorly assorted and 
almost completely oxidized character of the sediments; the frequent 
occurrence of ripple-marks and mud-cracks, and the fact that the matrix 
of the basal conglomerate on the north shore is frequently limestone in 
such a condition as to suggest subaerial deposition similar to that 
occurring in the Bighorn Mountains at the present day. 
The origin of the Keweenawan igneous rocks is assigned to fissure 
eruptions in the vicinity of the Lake Superior basin caused by orogenic 
movements and the down-warping of the basin apparently began in the 
Middle Keweenawan epoch. The source of the sediments is found 
chiefly in the underlying igneous rocks and the maximum thickness of 
the series is regarded as not more than half of that which was assigned 
to these rocks by some earlier writers. The discrepancy is due to the 
consideration that they were deposited on an inclined surface. 
