128 THE MAEQUETTE lEON-BEAKISG DISTRICT. 



ores associated witli these. No new facts ai-e given with respect to the 

 origin of the orc^s, Lnt the author repeats his former view and emphasizes 

 the notion of the eruptive oi'igin for these particular ores. 



Al)ove the ores, and forming the base of the succeeding Repuldic for- 

 mation, he ])hices a series of fragmental deposits of ore and jaspilite. These 

 are described at mucli greater length than in the earlier article. The ores 

 constitute a large proportion of those mined. They ])ass upward into 

 ([uartzites. The contacts between the fragmental and eruptive jaspilites^and 

 ores can be seen at the Jackson, Lake Superior, and other mines. In the 

 Cascade range all the jaspilite is of the fragmental kind; its layers are 

 interlaminated with those of quartzite. 



Above this fragmental jaspilite foi-mation, with its associated qviartzites 

 and schists, both at Republic and at Cascade, there is another fragmental 

 series, composed of conglomerates, (piartzites, schists, etc. This formation 

 is well seen at the Holvoke mine, and hence has been called the Holyoke 

 formation. 



'Die diabases and diorites are still believed to liave been erupted after 

 the denudation of the nonfragmentai jaspilites — that is, in Republic time. 

 Of the granite, however, two eruptions are now recognized. ()ne of these 

 "is older than tlie detrital jaspilite, if not older than any of the ore-beariug 

 formations, while another occurred subsequently to the ei'uptions of the 

 beforementioned * * * dialiases and diorites." 



The peridotite of Presque Isle is now regarded as younger than the 

 granite in its proximity, while that northwest of Islipeming is shown to be 

 younger than any other of the large intrusive masses in tlie jMarquette 

 district. 



The most important facts brought out in the paper, especially impor- 

 tant because the}" are opposed to the author's earlier opinions, are: the 

 divisibility of tlie Azoic into three formations of diHerent geological ages; 

 the recognition of an old granite, older than most of the ore deposits; and 

 the acknowledgment of the sedimentary origin of many of the ores and 

 jaspilites. T\w green schists are j)resumabh' still placed above the oldest 

 formation of the district. 



