Page:The Journal of geology (IA journalofgeology21894univers).pdf/244

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, contains 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 designated by him as cherty iron carbonate. One of the principal points which Van Hise brings out in the discussion of his theory for the formation of these deposits is that the change has been largely an oxidation 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 section, the iron deposits are referred to as "iron ore, replacing ferruginous 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 geological 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.