Geology and Mineralogy. 287 



Chapters XVII, XVIII, XIX discuss the iron, copper, and silver 

 ores, W. J. Mead cooperating in the chapter on iron ores ; and 

 chapter XX gives a resume of the general geology. 



From the mass of valuable material contained within this mon- 

 ograph may be singled out for special mention a few points of 

 novel interest. As a result of studies of a theoretic nature and 

 laboratory experiments in addition to field observations, it is be- 

 lieved that the irou ores were mostly deposited as marine forma- 

 tions and the iron salts rapidly extracted from lavas while they 

 were still hot, partly from juvenile emanations. Ordinary surface 

 weathering seems to have played but a small part in the initial 

 concentration. In regard to the formation of greenalite the chem- 

 ical studies are especially instructive, as it was found possible to 

 produce artificial greenalite. The reaction of salt water upon hot 

 basalts or porphyries results in sodium silicate. This reacts 

 directly with soluble ferrous salts, for instance FeS0 4 , and pro- 

 duces a mixture of ferrous silicate aud free silica. If a soluble 

 magnesium salt is also present it is added to the precipitate, 

 explaining the presence of magnesia in greenalite. This was the 

 dominant original mineral and is still abundant, but secondary 

 concentrations have resulted from oxidizing and hydrating of the 

 iron minerals in place, leaching the silica, and convergence of 

 iron carried temporarily in solution. The hematite and magne- 

 tite ores are the end results. 



Under the chapter on general geology may be noted the dis- 

 cussion as to the dominantly igneous character of the Archean. 

 It was followed by a period of uplift and profound erosion which 

 planed across the older structure. Upon this was laid down the 

 Lower Huronian, regarded by the authors as a terrestrial deposit 

 north of Lake Superior, as a series of marine formations on the 

 south. Following this period and separated from it and from 

 each other by great unconformities came the Middle and Upper 

 Huronian considered as dominantly marine, and the Keweenawan, 

 looked upon as dominantly terrestrial. The characters of many 

 formations change in passing from south to north, and the authors 

 regard this as of probable significance in the great development 

 of the iron ores in this region rather than over the great Cana- 

 dian shield. " The Lake Superior region, then, may be regarded 

 broadly as a part of a great metallographic province containing 

 a variety of ores associated with volcanism, which may be associ- 

 ated with folding along an old shore zone." j. b. 



2. The Elastic- Rebound Theory of Earthquakes ; by Harry 

 Fielding Reid. University of California Publications, Bulletin 

 of the Department of Geology, vol. vi, No. 19, pp. 413-444, 1911. 

 — In this text, elaborated from a lecture given at the University 

 of California, Professor Reid presents in briefer form that theory 

 of the movement causing the California earthquake which he has 

 published in the report by the Carnegie Institution. He here 

 gives it a greater degree of generality and shows how certain 

 other earthquakes conform to the theory of a slow progressive 





