PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 367 



Iron ore and slate.] 



3. The only other observable element is silica, of a form that is so fine grained that it is comparable to 

 the so-called chalcedonic silica of the jaspilyte of the Vermilion Iron range. It might be supposed that each of 

 these balls consists of a single mass of silica, having one orientation; but each ball is composed of many fine 

 grains, and similar fine grains also fill the interstices between the balls. 



4. In a high power, and the nicols crossed, these fine silica grains show their separate boundaries, and it 

 is observable that in many cases they have the clusters of dark particles as nuclei. Each silica grain embraces 

 one, rarely two, of these clusters where the clusters are numerous, but where they are wanting the silica grains 

 have no central dark cluster. Occasionally a dark cluster is separated from the rest and occupies the centre of 

 a grain in the midst of silica grains that have no nuclei. There is occasionally some confusion, and a cluster seems 

 to be divided between two silica grains. But in the main it is plain that the silica grains grew up in some way 

 under the influence of the dark clusters whenever the material of the clusters was present. 



The dust particles are so fine that in the thickness of the slide, cut less than .03 millimeters in thickness, 

 many particles overlap and interfere with each other so as to produce, in the otherwise clear quartz, a grayish 

 obscurity. By lowering the objective, after the first particles come into view (in natural light), others are seen, 

 but, owing to the effect of those that lie above the focal point, as well as to others that still lie below, they cannot 

 be clearly differentiated from the obscurity; yet it is possible in many cases to discern the shapes of the dust 

 clusters; hence, as seen in figure 21, 



5. These clusters are rarely or never angular, though frequently they have one part sharper than the 

 general rotundity. They are so constructed, sometimes, as to show that the particles are arranged in a manner 

 of a cylinder or tube. This is evident by a proper manipulation of the fine adjustment screw, viz.: After the 

 objective is lowered so as to focus on the upper side of the section, the particles that are distinctly in focus form 

 perhaps a circular enclosure, or an oblong figure. By lowering the objective by a very small turn, the ring 

 disappears, but its form reappears at the lower focal point, with greater or less distinctness. If the cylinder be 

 crooked so that one side or end protrudes to the right or left from the focal point, the lowering of the objective 

 brings into view distinctly the whole length of the tube on one side as different parts come into focus, until it is 

 cut off by the lower surface of the section. 



In other cases, indeed, in most of the clusters, there is no semblance of any structure, but frequently the 

 clusters themselves are grouped, separated by transparent boundaries of siliceous matter in a manner that simu- 

 lates the cells of foraminif ers cut at hazard by the section. Indeed, they suggest strongly the cellular structures 

 represented by Messrs. Woodward and Thomas on plates C and D of volume i of this report. It seems as if the 

 clusters may have been the cells in which gathered the greensand element, and that on conversion from the 

 protoxide to the sesquioxide the iron particles were forced to take such distribution as the walls of the apart- 

 ments allowed, the walls themselves being replaced by silica. The general aspect of the magnetite, in its distri- 

 bution in the thin section, suggests the debris of foraminiferal cells, with only occasionally a form preserved that 

 exhibits a probable organic arrangement. 



The illustrations that are given by Clark (Geol. Survey of New Jersey, 1892) 

 showing the manner of accumulation of glauconite in the cells of foraminifers, 

 might be supposed to show the original forms of the glauconite grains from which 

 these clusters of magnetite powder were derived, but these are much finer than those. 



The first suggestion of the glauconitic origin of the iron ores of the Mesabi 

 range was made by Mr. J. E. Spurr, in Bulletin x, of the Minnesota Survey, and he 

 likewise first conjecturally referred the greensand to the agency of foraminiferal 

 organisms. His material was from the western end of the range where the ore is 

 hematite, and where much of the glauconite is yet in the amorphous protoxide state. 

 He found no structural evidence of organic agency in the microscopic slides which 

 he examined. A re-examination of the same slides, while revealing much amorphous, 

 though globular, greensand, yet does not give any suggestion of composite cellular 

 arrangement. It seems that all the slides examined by him were made from rock in 

 which the glauconite grains are isolated, each from the other, or are so broken and 

 recompacted into a formless mass that no organic order is preserved. In the eastern 

 end of the range the strata have been affected by the nearness of the gabbro range, 

 and at the same time, fortunately, the separate cells are not so completely isolated 



