GEOLOGICAL EXPLORATIONS AND LITERATURE. 59 



This report, as also later ones by the same author (for ISSl and 1882), 

 and by his successor, A. P. Swineford (for 1883 and 1884), contain brief 

 references to the results of private explorations for iron in. the Gogebic 

 reg-ion. They contain no geological information of significance. Actual 

 mining developments first began in the Gogebic country in the fall of 1884, 

 and the first shipments of ore of importance were made in the summer of 

 1885. 



1883. 



Chamberlin (T. C). Geology of Wisconsin, vol. i, ISS,'}, ])t. i. General 

 Geology, with general geological map of Wisconsin. 



In chapters iv, v, and vi of this volume Prof. Chamberlin gi\'es a 

 general account of the older rocks of Wisconsin, which, in accordance 

 with previous publications of the Survey, are regarded as divided into 

 three distinct formations — Laurentian, Huronian, and Keweenawan. 

 While in a considerable measure this account is a summary oi' conclusions 

 already amiounced in previously published volumes of the Geology of 

 Wisconsin,' it still embodies some later conclusions, as also the most com- 

 prehensive statement of Prof. Chamberlin's own -^n'ews on these formations 

 yet published. The following extracts will serve to show what these 

 views are: 



(1) As to the Laurentian: 



Synopticul notes on Laurentian formation. — Name derived from Laiirentide lulls 

 of Canada. Rocks of metamorpliic class, mainly gneisses. Thickness undetermined, 

 but great. Strata much folded and contorted. Occupies a large area in northern 

 Wisconsin. .■ . . 



General character of the rodcu. — We have already referred to this as the granitic 

 foundation upon which the rock structure of our state is builded. Tlii^ rocks as we 

 now tind them consist of a series of granites, . . . gneisses, . . . syenites, 

 . . . hornblcndic, micaceous, and chloritic schists and allied rocks. With tliese 

 are associated igneous diabases . . . and similar rocks, together with diorites 

 . . . of undetermined origin. Among these rocks the gneissoid granites vastly 

 predominate, so that tlie whole series in a general view is conveniently termed 

 granitic. 



Sedimentary origin. — But throughout the series evidences of sedimentary 

 accumulation abound: (1) in the foliations and stratification, (2) in the alternating 



' Vols. II, III, and iv of this series iintednte vol. i in time of publication. 



