THE IKON-BE AEING MEMBER. 229 



]>osefl of an a]>haiiitic flint; the dark fjray bands, wliilc containinij more or less r)fThis 

 tlint, appear t« be made up ofa crystalline aggregation of some carbonate, the crystal- 

 lization being coarse enough for the naked eye to detect the cleavages of the indi- 

 vidual crystals. Sp. gr. of 9009, 3-24. 



In thin sections the black hands of the rock represented l>y tliese sixH'imens prove 

 to be made up mainly of an amor[)hous and an exceedingly tinely (aystalline silica. 

 In this siliceous background are included numerous minute black particles, to which 

 evidently the black color of tlic bands is due. Tliese jiarticles, when examined with 

 a high power, are found to be exceedingly irregular in shape, though often thej' are 

 aggregated into thin belts, which arc so plentiful as to give a strongly laminated 

 appearance to these portions of the sections. They appear to be quite without crys: 

 tal outlines, and from tlie macroscopic appearance of the rock and its close resem- 

 blance to others in which carbonaceous matter has been detected on analysis it is 

 supposed that these jiartides are of that nature also. Very possibly they luay be 

 mingled with more or less of pyrite and iron oxides. A few pieces of what ai)i)ears 

 to be a fragmental quartz are met with in those portions of these sections which 

 represent the black bands, and also a few irregular areas of iron carbimate which 

 are analogous in structure and alterations to those sjioken of under .'52. The (/raj/ 

 bands seen micro.scopically have a background or groundmass similar to tin; material 

 which composes the bulk of the black bands. But in this case there are contained iu 

 this groundmass, so plentifully as to constitute the larger ]iorti(>n of these parts of 

 the sections, ii-regular areas of the iron carl)onate in various stages of alteration and 

 others less plentiful of a greenish chlorite. These greenish areas have often an oval 

 form. The viridite or chlorite which makes them up is arranged in fan-like aggre- 

 gations. The ai-eas of iron carbonate vary greatly in size, tlie smaller ones being 

 almost perfect single rhomboliedra; while the larger ones, although aggregates of a 

 number of individuals, show around their borders the projecting edges of rhom- 

 bohedra. Much of this iron carbonate has altered more or less thorougldy to iron 

 oxide.s, including botii the brown oxide and hematite. In many cases wln-re the 

 alteration has been complete the secondary iron oxides are plainly jKufectly ])seudo- 

 morphous after the siderite, i. e., they presei-ve still the rhombic outlines of the 

 siderite crystals. More usually, however, the alteration has been only partial, and 

 in such cases tlie secondary oxides are either in irregular bunches or (and this is 

 very much more connnonly the case) are arranged iu very irregulai- concentric 

 spherical rings, of which there are generally several in one area, though in other 

 cases a single ring is seen or a single ring with the beginnings of one or more 

 others. These rings of iron oxide, which are so plainly the result of a secondary oxi- 

 dation of the carbonate, occur quite without any reference to the individuals of the 

 latter nuneral; that is to say, a single ring or a set of rings traverses an area 

 made up of a number of cariionate individuals. In some of the most completely 



