46 THE MARt^UETTE lltON-BEAlilNG DISTRICT. 



replace them elsewhere. lu some of the mines, as at the Lake Superior 

 and the Cleveland, the ores are closely associated with octahedral crystals 

 resembling magnetite, except that they possess a red streak. The magnetites, 

 hematites, and limonites are believed to represent different stages of devel- 

 opment in the alteration of the same mineral substance. The hematite is 

 regarded as an oxidation product of magnetite, as is indicated by the pseu- 

 domorphs of the former after the latter mineral in the chlorite-schists. The 

 limonite is hydrated hematite. As for the origin of the magnetite, it is 

 thought j)ossiljle that this was precipitated from a solution produced by the 

 action of carbonated waters on carbonate of iron, and that the entire cycle 

 of changes from the carbonate to limonite was completed before the begin- 

 ning of Cambrian time. 



The lower portion of the Huronian, represented, according to the 

 author's view, by the "chlorite-schists" north of Marquette, is cut by dikes 

 of dioritCj and north of Light-House Point by a dike of red syenite con- 

 taining fragments of diorite, uphanite, quartzite, ehlorite-sehist, and hematite. 

 These diorites and the syenites, he declares, are the only eruptive rocks 

 observed by him in the iron-bearing- region. 



The chlorite-schists referred to are not now regarded as members of 

 the iron-bearing series, so that the Jiorites and the syenite cutting them are 

 not necessarily younger tlian the iron series. The syenite dike containino- 

 fragments of quartzite and hematite has not been seen by any one but 

 Credner. 



A geological map and several geological sections accompany the article. 



The work on the Marquette district up to the close of the sixties and 

 the estaldishment of the geological survey of Michigan had been concerned 

 with the general relations of the rocks to one another and their separation 

 into large groups or series. Foster and Whitney had succeeded in differ- 

 entiating the pre-Cambrian rocks from later formations, calling the former 

 the Azoic series. The granites associated with the Azoic rocks were claimed 

 to be intrusive into these. The greenstones so abundantly present in the 

 Marquette area were regarded as metamor})hi(^ rocks, with the exception of 

 the small, well-delined dike masses, and the jasper ores were thou<>-Iit to 



