JASPILITES. 831 



quartz. Each grain may become coated with a film of hematite, thus 

 giving it a red color. Some of the more irregularly banded rocks have been 

 called cherts with bands and shots of ore. In these rocks minute openings 

 exist between the particles, and minor geodal cavities are very common, 

 thus giving the rock a porous character. This porosity results in part from 

 the diminution in volume in the change from iron carbonate to hematite or 

 limonite. Also it is partially caused by the solution of the calcium and 

 magnesium carbonates and silica more rapidly than iron oxide is substituted 

 for these materials. During the process the dissolved iron carbonate may 

 be transported a greater or less distance. In openings precipitation may 

 take place, and thus veins of hematite or limonite be produced. Under 

 favorable conditions, which are more fully discussed in Chapter XII, on 

 "Ore deposits," the excess of silica and other material may be dissolved 

 and nearly pure hematite or limonite bodies be formed. Thus from the 

 iron carbonate large iron-ore deposits are produced. The details of the 

 process of transformation I have fully discussed elsewhere. a 



In closing this part of the subject it may be noted that to the stage 

 where the ferruginous shales and ferruginous cherts are produced the iron 

 has been twice in the form of the carbonate, or once as a carbonate and 

 once as a silicate, and twice in the form of an iron oxide, and, therefore, 

 that at least four chemical transformations have occurred to that time. 

 These were: (1) When the material was first taken into solution as car- 

 bonate, (2) when it was precipitated in the sea as hydroxide, (3) when 

 it was transformed to an iron carbonate or hydrous iron silicate, and (4) 

 when by the underground waters it was again chemically changed to 

 limonite or hematite. 



JASPILITES. 



The jaspilites are banded rocks, the bands being composed of hematite 

 and chert in various proportions. The jaspilites differ from the ferruginous 

 cherts in that the siliceous bands have a bright-red color, due to the fact 

 that each granule of quartz is coated with and includes innumerable minute 

 flakes of blood-red hematite. The bands in which the iron oxide is pre- 

 dominant are composed chiefly of specular hematite. 



"Irving, R. D., and Van Hise, 0. R., The Penokee iron-bearing series of Michigan and Wisconsin: 

 Mon. U. S. Geol. Survey, vol. 19, 1892, pp. 268-295. Van Hise, C. R., The iron-ore deposits of the 

 Lake Superior region: Twenty-first Ann. Rept. IT. S. Geol. Survey, pt. 3, 1901, pp. 326-328. 



