32 TDE MARQUETTE IRON BEARING DISTRICT. 



fritpnK'utal rock is inipre'i'nateil witli tlie oxide. This observation leads 

 the authors to coiicUkU! that the dittiisiou of ore through tlie rocks must he 

 ascribed to some general cause quite independent of the nature of the rock 

 itself. Many descriptions of now famous ore Ijodies are given. It is fre- 

 quentlv asserted that tlie ore l)eds are associated with the greenstone dikes, 

 and that the other rocks associated with the ores are saturated with emana- 

 tions of iron oxide. In sees. 10 and 11, T. 47 N., R. 27 W., in the vicinity 

 of the present citv of Ishpeming, large deposits of almost pure ore were 

 discovered. Here the ore "exhibits many of the characters of an igneous 

 eruptive rock, and can not be regarded in any other light than as a huge 

 lenticular mass, which has been elevated to its present position from beneath 

 while in a semifluid state, exactly in the same way as the trappean ridges 

 wliicli accomjjany it and which it so strikingly resembles in general outline 

 and position." After discussing- the various tlieories that might be offered 

 to explain the existence of ore and jasper in the forms noted in the region, 

 the authors conclude witli a resumed of their opinions with respect to the 

 origin of the Azoic rocks in general and the ore beds in particular. We 

 quote the resume entire (p. 69): 



Wc may conceive that the various rocks of the Azoic series -svere originally 

 deposited in a nearly horizontal position, at a period prior to the appearance of organic 

 life upon the earth ; that these stratified deposits were composed, for the most part, 

 of finely comminuted materials, principally siliceous and argillaceous, in some cases 

 consisting of almost pure silex, like the purest portion of the Potsdam sandstone 

 which was afterwards deposited upon these strata. 



During the dejiosition of these strata, at various intervals, sheets of jilastic 

 mineral matter were poured forth from below and spread out upon the surface of the 

 preexisting strata. These igneous rocks are exceedingly compact and uniform in 

 their texture, which would seem to indicate that they were under heavy pressure, 

 probably at the bottom of a deej) ocean. The same depth of water is also inferred 

 from the comparative absence of ripple-marked surfaces throughout the whole series. 



During this period the interior of the earth was the source of constant emana- 

 tions of iron which appeared at the surface in the form of a plastic mass, in combina- 

 tion with oxygen, or rose in metallic vapors, or as a sublimate, perhaps as a chloride; 

 in the one case it covered over the surface like a lava sheet; in the other it was 

 absorbed into the adjacent rocks or difiused through the strata in process of formation. 

 Besides, a large amount of iron entered into the composition of the igneous rocks of 

 this period, chiefly in combination with silica, as a .silicate of the protoxide. Portions 



