WEATHEEING OF CHEETS 215 



being an average of two analyses of the black, little altered 

 material from the interior of one of these blocks, and II that of 

 the residual clay. In III, IV, and V are given the calculated 

 losses of constituents, as before.^ 



This residual clay, when boiled with hydrochloric acid and 

 sodium carbonate solutions, yielded up nearly 70% of its matter 

 to these solvents, leaving a residue which, when examined under 

 the microscope, shows only faint yellow-brown scale-like par- 

 ticles, rarely over a tenth of a millimetre in diameter, acting 

 very faintly, if at all, on polarized light, and with borders often 

 serrate, through corrosion, though this latter feature may be 

 due, in part, to the action of the solvents used. The mean an- 

 nual temperature of the region is 52.3° Fahr., and the rainfaU 

 some 47.9 inches. 



Weathering of Chert. — Among siliceous sedimentary rocks 

 poor in alkalies or iron-bearing silicates the degeneration is 

 mainly disintegration, though a small amount of silica, existing 

 in either crystalline or chalcedonic forms, is usually lost through 

 solution. Thus the cherts of southwest Missouri break down into 

 porous friable forms, sometimes passing into the condition of 

 loose powder, or again retaining sufficient tenacity to be utilized 

 for filter discs and tubes, as at Seneca, in Newton County. 



Analyses of fresh and altered forms of this material, as given 

 by Dr. B. 0. Hovey,^ show no differences that are of sufficient 

 importance to warrant us in assuming any of them as the direct 

 cause of disintegration. The change is evidently mainly physical, 

 though it is more than probable that a certain amount of 

 interstitial silica has been removed. It is, of course, possible 

 that here, as in other forms of decomposition, extensive solution 

 may have taken place, leaving a residue which, so far as compo- 

 sition is concerned, gives no clew to the changes which have 

 occurred. Dr. Penrose, however, describes® a process of chert 

 decay, or more properly disintegration, as manifested in the 

 Batesville rtegion of Arkansas, in which the cause of the break- 

 ing down is more apparent. There are two stages in the process, 

 as described: (1) A transition into a light, porous, opaque, 

 buff-colored rock of the consistency of ordinary pressed brick, 

 and (2) into an impalpable white or brown powder, locally 



* An analysis of a perfectly f resli slate from tliis locality is given on p. 119. 

 = Appendix A, Vol. VII, Missouri Geological Survey, 1894, pp. 727-739. 

 3 Ann. Eep. Geol. Survey of Arkansas, Vol. I, 1890. 



