PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 595 



Jaspilyte.] 



Mic. The section shows the usual jaspilyte, except that the iron ore is more 

 abundant than is commonly the case and that there is much magnetite in crystals. A 

 noticeable feature of the section is the presence of many small, colorless veins. 

 These are composed of minute grains of quartz similar to the quartz of the usual 

 jaspilyte, thus showing that what has been termed "chalcedonic silica" in the 

 reports of this survey does form in veins. 



One section. Compare No. 903. 



Age. Keewatin. u. s. G. 



No. 903. JASPILYTE. 



Stone mine, Soudan. 



Ref. Annual Report, xv, page 389; Annual Report, xvii, pages 194, 215. 



Meg. Similar to No. 902, except for two things: (1) In this specimen the 

 magnetite crystals occur to some extent in the red bands, while in No. 902 they are 

 confined to the black bands; (2) No. 903 has a few pyrite crystals; these seem to be 

 confined to the red bands. 



Mic. The usual jaspilyte with very abundant iron ore, some 'of which is 

 magnetite in crystals. The black bands are composed of quartz, of coarser grain than 

 in the red bands, of less hematite, and of many coarser crystals of magnetite. There 

 are also, as in No. 902, veinlets of fine-grained quartz cutting the section. Consid- 

 erable of a very strongly doubly refractive mineral calcite or siderite, most 

 probably the latter is present. Two sections. 



Age. Keewatin. u. s. G. 



Remark. The fine sideritic element is in granular microscopic patches distrib- 

 uted amongst the fine quartz and hematite. It also sometimes surrounds the 

 magnetite crystals or adheres to them. It is fresh and non-limonated. It is 

 coarsest in those parts of the slide where the magnetite crystals are coarsest, and is 

 finest in the reddened (hematite) parts where the quartz is also finest. So far as can 

 be determined from this slide, these iron minerals, whether oxides, sulphide or 

 carbonate, stand on an equal footing as to date of origin. The veinlets of quartz cut 

 across all the other bands, and are necessarily later in date. In this slide, and in 

 that of No. 902, are traces of a green chloritic element which is apparently the same 

 as that which constitutes the bulk of the green schists which enclose the iron ore lodes. 



In the thinner section, made by Marchand, it can be seen distinctly, in high 

 power, and on lowering the condensing lens, that the sideritic groups embrace many 

 of the finer hematitic dark grains, or dust particles, and also that frequently such 

 dust particles seem to serve as central nuclei of the individual siderite grains. But 

 sometimes several such dust groups are embraced in a single siderite grain. 



N. H. w. 



