430 THE PENOKEE IRON BEARING SERIES. 



rock is doubtless the cause of the mingling of clastic and nonclastic sedi- 

 ments which is so characteristic of the exposures just to the north and less 

 marked in the exposures in Sees. 21, 22, 23, T. 47 N., R. 43 W., Michigan. 



After the deposition of a thin layer of nonclastic sediments witli the 

 moditicatious indicated, but long before the cessation of the nonfragmental 

 sedimentation <>f the Iron-bearing member of the western area, came the 

 great volcanic outbreak which resulted in ])iling up the series of eruptive 

 flows, tuflPs and interstratiiied sediments of the greenstone -conglomerate area. 



The truly aqueous interstratiiied rocks doubtless received nmch of 

 their material fntm the contemporaneous volcanics. This group of forma- 

 tions is then very similar to the central portion of the Keweenaw seiies, 

 consisting of basic lava flows, with interbedded sedimentary rocks.^ 



Evidently the center of the volcanic activity was the E. .J of T. 47 N., 

 R. 44 W., Michigan. To the east this material reaches no farther, or little 

 farther than the NW. \ of Sec. 23, T. 47 N., R. 43 W., Michigan. This c.m- 

 paratively short eastern extent of disturbance may have been due to the 

 fact that the higher lands to the east prevented tarther extension ; for, as has 

 been seen, all of tlie Eastern area rocks east of this point very probably are 

 at a higher lioriKou than we have vet reached. To the west, the influence of 

 the volcanic activity extended farther. The accunuilation of the ^■olcanic 

 outflows and of the greenstone-conglomerates went on with great i-apidity 

 as compared with the exceedingly slow process of nonclastic sedimentation 

 which was continuing to the west in the iron-bearing area. As explained, 

 this accunuilation was accompanied by siauiltaneous sinking of the area 

 thus l)urdened. 



If the above conclusions are correct, a shore line existed not far to the 

 east during the time of the deposition of the Quartz-slate, Iron-bearing and 

 Greenstone-conglomerate belts. It follows that the volcanic center was prob- 

 ably submarine at tlie inauguration of the igneous action, and was near 

 a shore line, as is usually the case with existing volcanoes.- 



I K. P. Tivinir: Coitppr-licarin}; rocks of Lake, Superior. Men. V, U. S. Geol. Survey. 



^Since the iiliiive w;is written, I'lirtlier tield work north of Crystal Kails, Miehifjan, has shown 

 the existen<e nl an ixtensive area, iilenti(^al in most, respeets with the greenstoue-eimmlipnierate groii]> 

 of the KastiTn area, i. i-., it consists of <lial)ase, |)orphyrites, aniygtlaloids, greeu.stone-confrlonicrates 

 and contenii>iiraiie()iiM s mdstones aud couglouiorates the material of which was mainly derived from 

 the contemporaneuns volcanics. 



