CONDITIONS AFFECTING RESULTS OF ANALYSES. 327 



From these analyscrf it would appear that of the 17 grains of silt, rep- 

 resenting 4 i)er cent of the total disintegrated material, only 89.7 per 

 cent is soluhle ; and, further, that a very considerahle proportion of the 

 insoluhle residue, as indicated by the high percentages of alkalies and 

 lime, still consists of unaltered soda lime and })otash feldspars, the iron 

 and magnesia alone having been largely removed. 



coyoiTioys affecting the results. 



These results are not quite what one would be led to expect from a 

 perusal of the literature bearing upon the subject of rock decomposition. 

 As long since noted by J. G. Forchhammer, G. Bischof, T. Sterry Hunt 

 and others, the ordinary processes of decay in siliceous rocks containing 

 ferruginous protoxides and alkalies consists in the higher oxidation and 

 separation of the i)rotoxides in the form of hydrous sesquioxides and a 

 general hydration of the alkaline silicates, accompanied by the formation 

 of alkaline carbonates, which being readily soluble are taken away nearly 

 as fast as formed. More or less silica is also removed, according to the 

 amount of carbonic acid present — a portion of the alkalies forming solu- 

 ble alkaline silicates when the supply of the acid is insufficient to take 

 them all up in the form of carbonates. The apparent anomaly here 

 shown is partially explained by examination of the various separations 

 with the microscope. Thus the low percentage of silica is found to be in 

 large part due to the fact that the residual quartz granules are in many 

 cases too large to i)ass the 120-mesh sieve, or, if passing, have been largely 

 separated in the process of washing. Further, it is found that the sifting 

 has served to concentrate the small epidotes in the fine sand, and a por- 

 tion of them have even come over with the silt. The presence of this 

 epidote also exi)lains in part the high percentage of lime shown, since 

 the mineral itself carries some 20 to 24 per cent, of this material. The 

 large i)ercentages of magnesia, soda and potash cannot, however, be thus 

 accounted for, and we are led to infer that either these elements are there 

 combined in minute amorphous zeolitic comj)ounds, unrecognizable as 

 surli under the microscope, or, as seems to me more pro]3al)le, the feld- 

 s])athic constituents to which the alkalies are to be originally referred 

 have undergone a mechanical splitting up ratlier than a chemical de- 

 composition. This view is to a certain extent borne out by micros('o})ic 

 studies, but it is difficult to measure by the eye the relative abun(hince 

 of tliese constituents with sufficient accuracy to enable one to form any 

 satisfactory conclusion. The magnesia must come from the shreds of 

 mica, many of which, from their small size and almost llocculent nature 

 when decomposed, would naturally be found in the silt obtained as stated. 



