100 THE MARQUETTE IKON-BEAKING DISTRICT. 



He declares that in Ixitli localities the rock was originally a Iherzolite, as he 

 liad already soiue time earlier declared the Presque Isle rock to be. The 

 limestones associated with the serpentine are also believed to be altered 

 Iherzolites. The peridotite is still believed to be intrusive in the overlj'ing 

 sandstones, and for the same reasons previously given. Fourteen thin 

 sections of rocks were examined. Their descriptions leave no further doubt 

 as to the correctness of the author's view concerning the origin of the ser- 

 pentine. A colored plate containing three lithographic reproductions of 

 the microscopic appearance of thin sections of Iherzolite, serpentine, and 

 dolomite illustrates the descriptions. 



Irving, R. D., aud Van Hise, 0. R. Ou secondary eulargenieuts i)f mineral 

 fragments in certain rocks. Bull. U. S. Geol. Survey No. 8, WashiTigton, 1884. With 

 plates of thin sections. 



Irving and Van Hise announce some of the results of their study of 

 thin sections of Potsdam and St. Peter's sandstones — results which had 

 already been aianounced in a brief form by Irving in an article published 

 in June, 1883, in the American Journal of Science — and add a number of 

 new facts concerning these and similar rocks of other horizons. The authors 

 show conclusively that many of the quartzites, quartz-schists, graywackes, 

 etc., of the Lake Superior and other regions are sandstones whose inter- 

 stices have been filled with secondary quartz, largely by deposition of this 

 material around the fragmental quartz grains in optical continuity with 

 their substance. This conclusion is important in that it indicates that a 

 part <;)f the quartzites and graywackes that make up a considerable pro- 

 portion of tlie Marquette iron-bearing series are not extremely metamor- 

 phosed rocks, Init are simplv sandstones hardened by (juartz infiltration. 

 Much of tile mica-schist of Brooks's lower quartzite, north of the Michigamme 

 mine, and of Formation XXI in the Penokee region shows enlarged quartz 

 o-rains, and the jaspers and cherts so common in the iron formation of the 

 Marquette Huronian sometimes contain enlarged fragmental grains of quartz 

 in a matrix of chalcedonic silica. This latter fact is in opposition to the 

 view of an eruptive origin for these rocks, and in favor of that which 

 ascribes to them a sedimentary origin. A large number of thin sections 

 are described in detail. They all confirm the conclusions outlined above. 



