488 THE -MARQUETTE lEON-BEAKING DISTRICT. 



Clarksburg rocks, wliilu the former are believed to be mainly of Clarksburg 

 age, and to be the lower jjortions of the rock masses whose surface facies are 

 represented in i)art by the flows and tuifs that constitute the main mass 

 of the Clai-ksburg formation. It has been suggested by Lane that the 

 younger intrusives cutting the Micliigamme formation may in a similar 

 manner be dikes from the volcanoes that yielded the Keweenawan lavas. 



The Clarksburg "greenstones" are discussed in connection with the 

 other rocks of the Clarksburg series. 



SECTIOX I.— THE PKE-CEARKSBtJRG GREEIfSTOXES. 



The igneous rocks associated with the beds older than the Clarksburg 

 formation, and especially those in the iron-bearing formation, have been 

 very thoroughly discussed by the different geologists who have studied the 

 Marquette district, while the dikes of later age have scarcely been men- 

 tioned. Practically all the references to "diorites," "greenstones," " dio- 

 ritic schists," and " chloritic schists" that are met with in the litei'ature of 

 the district apply to the "greenstones" and schists in the iron formation, 

 and these, as has already been seen, were usually regarded as interleaved 

 sheets, or as beds of metamorphosed sediments. 



Structurally the pre-Clarksburg eruptives occur principally as dike- 

 like bosses or as dikes, although sheets and tuff beds are also known to 

 exist. 



The dike and the boss masses are very much more common than the 

 other structural forms of the greenstones, and are those that have hitherto 

 been studied most carefully. They may be divided into two classes — 

 those occurring as typical dikes, and those forming bosses or boss-like 

 dikes. There are no essential differences between the rocks of the two 

 classes, except that the dike masses have been much more completely 

 altered than the boss masses. 



The boss masses form the large knobs of "greenstone" or "diorite" 

 that are so prominent a feature of the topography in the neighborhood of 

 Ishpeming and Negauuee. (Atlas Sheets XXVIII and XXIX.) Some of 

 these knobs may be regarded as parts of very large dikes, as, for instance, 

 the knobs in the northwestern portion of Negauuee, which together form a 



