2 30 THE JOURNAL OF GEOLOGY. 



tion of a thin crust of carbonate of copper on its surface, but even this 

 does not always happen, and one of the remarkable features of the 

 Lake Superior region is the very extensive occurrence of copper in its 

 native state. Copper sulphides are disseminated through the region, 

 and are probably the source from which the native copper was derived 

 in nature; but they have not been found to be themselves concentrated 

 in commercially important quantities, and are therefore not mined. 

 Small quantities of oxide of copper also occur, but are likewise not of 

 present importance. 



On page 125 Professor Tarr, in describing a section by Van Hise, 

 showing the mode of occurrence and formation of the iron deposits 

 in the Penokee-Gogebic range in the Lake Superior region, states that 

 Irving and Van Hise have shown that the hematite deposits of that 

 region were formed by a replacement of "beds of dolomitic limestone." 

 It may be said here that the iron deposits of the Penokee-Gogebic 

 range occur in the Upper Huronian series, which, in this district, con- 

 tains no dolomitic limestones. A dolomitic limestone occurs near the 

 base of the Lower Huronian of the district, but it has no connection 

 whatsoever with the Penokee-Gogebic iron deposits. Van Hise clearly 

 states, in his various publications on this subject, that the iron deposits 

 of the Penokee-Gogebic range represent a replacement of a siliceous 

 rock containing carbonate of iron and other carbonates, and desig- 

 nated by him as cherty iron carbonate. One of the principal points 

 which Van Hise brings out in the discussion of his theory for the for- 

 mation of these deposits is that the change has been largely an oxida- 

 tion of the iron carbonate and a replacement of silica by oxide of 

 iron. Professor Tarr also gives a geological section illustrating the 

 occurrence of the Penokee-Gogebic ores, and designated by him as 

 "modified from Irving and Van Hise." In the legend below the sec- 

 tion, the iron deposits are referred to as "iron ore, replacing ferru- 

 ginous chert" — a statement not at all in accord with Professor Tarr's 

 text just cited. It is, moreover, difficult to understand on what basis 

 an author, who has never studied a region, has "modified" the geo- 

 logical sections of other authors who have spent years in investigating 

 that region. 



The errors in this book that have already been pointed out are only 

 a few among the many that might be mentioned, but they serve to 

 show the want of familiarity with the subject and the inaccuracies 

 prevalent throughout the volume. 



