GEOLOGICAL EXi'LOKATIO^S AND LITEKATUKE— 1882. 93 



Tlit'iv are nuuiv varieties dt" these schists, t'rniii ahiiost l)lack to silver- 

 white ill eolor. Some are garnetiferous : others contain aiidahisite. All are 

 regarded as sedimentary. 



This concludes the author's observations on the iroii-liearing series. 

 The remainder of the report is devoted to the serpentines and the i-rup- 

 tive dikes met with in his explorations. He descril)es for the first time in 

 detail the serpentines northwest of Ishpeming. Both the Ishpemiug and 

 the Presque Isle serpentines occur in " noiistratitied masses, which, if tliev 

 ever originated from mechanical sedimentary deposits, are by chemical 

 action so completely transformed as to efface all traces of their former 

 detrital structure. They resemble a volcanic erujitive rock, forced to the 

 surface in a soft plastic condition" (p. 135). The dolomitic and other 

 phases of the rock are all accurately described, but nothing is added to 

 our knowledge of its age or origin. 



The eruptive dikes cutting the Huronian deposits, including the granite, 

 consist, in the order of their age, of diorites, dolerites, and certain schistose 

 rocks, proliably originally diorites. The diorite dikes vary in width from a 

 foot to 50 or (iO feet, the wider ones being, as a rule, coarser than the narrow 

 ones. The dolerites are found only in the highest formation. Most of these 

 are massive, but many of them, cutting granites, are schistose thrciugh 

 pressure. 



The quartz and other vein rocks of the district are described as fissure 

 veins. 



1882. 



Columbia University. The Marquette iron region. By the students of the 

 Summer School of Practical Mining, Lake Superior, 1881. Scliool of Mines Quarterly, 

 Vol.III, 1882. I, November, 1881, pages 3.5-48; II, ibid., January. 1882, pages 103-117; 

 III,' ibid., pages 197-207; IV, ibid., pages 213-253. 



In the summer of 1881 the students of a class in the Summer School 

 of Mining connected with Columbia University spent a fe^^' weeks in the 

 Marquette district, and in the following year published a record of their 

 observations in a series of articles. Parts III and IV of the series and a 

 portion of Part II are devoted exclusively to descriptions of the mining 



