DEPOSITION OF THE LOWER MARQUETTE SERIES. 561 



being diagonal, ruiniing in a northwest -soutlieast direction, still furtlier 

 suggesting what was said at first, that the advance of the seaAvas from tlie 

 northeast. The subsidence continvied faster than the upbuilding of the 

 sands, so that there followed above them inud deposits, which have been 

 compacted into the Siamo slate. During the time of mud deposits the shore- 

 line continued to advance, and before this formation was completed the 

 sea had entirely overriden the Marquette district, with the possible excep- 

 tion of the southwestern part. Following naturally from the conditions 

 of deposition, the Siamo slate has a greater thickness in the eastern than 

 in the western part of the district, and it does not appear in the southwestern 

 part. Perhaps equivalent to some part of the Siamo slate in age is the 

 basal quai-tzite from Humboldt to Republic, but as it is impossible to say 

 what part of the quartzite belongs with the Siamo slate and what part 

 with the Ajibik quartzite, it is all mapjied as the latter formation because of 

 its lithological likeness to it. 



The steady subsidence during the deposition of the Siamo slate so 

 increased the depth of water that a nonfragmental formation began to 

 be deposited. This Avas the siderite slate, which has been largely trans- 

 formed into the varieties of rocks of the iron -bearing formation. The 

 conditions which led to the deposition of the iron carbonate are not certainly 

 known. At that time the Marquette transgression had entirely overridden 

 the land of the district, but it is not probable that all adjacent land areas 

 had disappeared, or even that the green schists of the Northern Complex 

 were entirely covered by the sea, although it is possible, or even probable, 

 that the long-continued erosion had reduced the land areas nearly to base- 

 level, and consequently that chemical solution, rather than meclianical 

 wear, was the more important agent of erosion. Thus might be explained 

 the large amount of iron salts which appeared. Doubtless the supply of 

 ferruginous material was in the form of iron carbonate, taken into solution 

 by direct atmospheric agencies, perhaps with the assistance of organic 

 acids. The basic eniptives of the Basement Complex, and especially the 

 surface volcanics on the northern border of the district, are very rich in 

 iron. These latter, being tuffs and lavas, were porous, and perhaps from 

 them came the greater proportion of iron. In the water, also, there was 

 MON xxviii 30 



