PETIIOGKAPUICAL CIIAKACTElt OF NEGAL:NEE FOKMATIOX. 371 



processes were so long contiuued within the ferruginous cherts that it could 

 not be expected that residual siderite should occur, yet in one or two cases a 

 small quantity is found. However, in the field the gradations are so com- 

 plete that one can not doubt that these rocks were i)roduced by the altera- 

 tion of an original sideritic chert, combined with secondary infiltration. It 

 is highly probable that much of the iron oxide and nmch of the silica now 

 present were derived from an iron carbonate once above the ferruginous 

 cherts, but now removed by erosion. The ferruginous slates represent 

 the kinds of rock produced by the simple oxidation in place of the orig- 

 inal sideritic slates, and from them, by the secondary actions described, 

 are produced the ferruginous cherts As in the case of the sideritic slates 

 and the griinerite- magnetite -schists, fragmental material is occasionally 

 recognized. 



In thin section the jaspilites (Pis. XXIV-XXVII) have a minutely 

 lanfinated character, each of the coarser bands, as seen in hand specimen, 

 being composed of many laminae, due to the irregular concentration of 

 the iron oxide. These laminae are of greatly varying width. The^' unite 

 and part in a most irregular fashion, producing a mesh-like appearance, and 

 frequently laminae disappear, as do the coarser bands. The complex, bright- 

 red jasper bands are composed mainly of finely crystalline cherty quartz, 

 but they are everywhere stained with minute particles of blood-red hema- 

 tite. The particles of quartz average less rather than more than 0.01 mm. 

 in diameter, and each of these minute grains contains one or more particles 

 of hematite. These are concentrated in laminas or are separate fiecks 

 included in the quartz grains. In some cases the hematite appears to be 

 somewhat concentrated between the grains, but in general it is arranged in 

 entire independence of them, as though it were present before the silica had 

 crystallized. The most ferruginous bands contain a predominant amount 

 of iron oxide, but in them is included much quartz, exactly similar to that of 

 the jasper bands. The original, translucent, red, mashed hematite is easily 

 discriminated from the secondary, crystal-outlined hematite and magnetite. 

 The folding, faulting, fracturing, and brecciation, spoken of in hand 

 specimen (Pis. XXV and XXVI), are beautifully shown under the micro- 

 scope. The resultant cracks and crevices are filled with secondary quartz 



