GEOLOGICAL EXPLOKATIONS AND LITERATURE— 1SS5. 103 



such circumstances that their oiigitial sedimentary deposition seems to be placed 

 beyond doubt. What has l)eeii the origin of the iron oxide of any of these ores, 

 whether fragmental or chemical, or both, I do not undertake now to discuss, but that 

 inucli of the quartzitic material mingled with them, particularly in the Penokee belt, 

 Las had the same fragmental origin with the associated quartzites I Lave convinced 

 myself from study in the field and from study of the thin sections. Besides this 

 fragmental siliceous material, however, and occurring frequently intermingled with 

 it, and again at times almost or entirely excluding it, is a chalcedonic or amorphous 

 silica. Much of the jasper of the Marquette ores seems to be made up of purely 

 crystalline quartz, but much of it also is chalcedonic or amorphous. * * * So far 

 as our study has extended it Las seemed evident to us that this chalcedonic silica is 

 of original formation, or at least that it existed in its present condition prior to the 

 formation of much of the series. 



Reference is made to the fact of the existence of fragments of ore and 

 jasper in the quartzite-conglomerate overlying- the ore formation, which, it 

 is admitted, proves that the ores are older than the quartzite, and tliat they 

 were in their present condition prior to the deposition of the quartzite, but 

 this is not regarded as proof that the ores and associated jaspers are 

 eruptive. 



At the time the paper was published work was being done on the 

 schistose and gneissic areas, of the district, but it had not progressed 

 sufficiently to allow of a statement regarding the relations of the rocks of 

 these areas to those of the IMarquette sediments. Tiie article concludes 

 with petrographical descriptions of the different rocks met with in the Lake 

 Superior region, among which are many from the Marquette area. The 

 serpentine of Presque Isle is found to be an altered peridotite, as Wadsworth 

 had shown. Wadsworth, however, believed it younger than the overlying 

 sandstones, while Irving regarded it as older. The Ishpeming serpentines 

 were found to b_ similar to tliose of Presque Isle. 



The examination of thin sections of the Huronian rocks showed 



l)lainly that — 



the rocks which form the bulk of the llurouian in all areas do not properly fall under 

 the head of metaraorphic rocks. Of the remaining rocks met with in these areas, the 

 various augitic and hornblendic geeenstones, peridotites, and felsitic porphyries I now 

 look upon as in all probability of eruptive origin. There remain to be accounted 

 for the various hornblende-schists, chloriteschi.sts, mica-schists, hydromicaschists, 



