188 EOCK DISINTEGEATION AND DECOMPOSITION 



upon this basis, a proceeding for which, so far as alumina is 

 concerned, we have already good authority/ This method will 

 be adopted, however, only with the siliceous crystalline rocks, 

 in which, for reasons noted later, the process of decomposition, 

 we have reason to suppose, is more complex than in calcareous 

 and magnesian rocks poor or lacking in the alkalies. The 

 entire discussion is one beset with great difficulties, since we 

 lack definite knowledge as to the exact processes which have 

 been going on and need constantly to guard against assump- 

 tions too hastily drawn or based upon insufficient data. Indeed, 

 any assumption based upon the results of chemical analyses 

 alone is likely to lead to grave error. We may be certain, how- 

 ever, that the estimates as thus obtained are invariably too low, 

 since it is not possible to conceive of decay in which even the most 

 refractory constituent is not carried away in appreciable quan- 

 tities, "Whether the iron or the alumina remains most nearly 

 constant must depend upon local conditions. 



If, then, in this particular ease, the iron in the form of FcoOg 

 is considered a constant factor, by proper calculation we obtain 

 the results given in column (IV) on p. 189, which represent the 

 proportional gain and loss of the various constituents of the rock 

 in passing from the condition indicated in column (I) on the pre- 

 ceding page, to that indicated in column (III). Such a com- 

 parison is instructive as showing not merely the relative loss and 

 gain, but also the total loss of material, in this case 13.79%, ac- 

 companied by a gain of 2.16%, in volatile matter. 



Such results are still far from satisfactory, and it is believed 



the tables will be more useful and instructive can we show the 



percentage loss and gain of each constituent as compared with 



the same constituent in the original rock. This can also readily 



be accomplished by a process the formula for which is given 



below/ and by which are obtained the results given in cloumns 



V and VI. 



^ Gr. Both, Allegemeine u. Chemische Geologie, 3cl ed. 



A 



' The formula employed in these calculations is as follows : ^^ ^ y^ — ^ '• 



and 100 — x = y, in which A = the percentage of any constituent in the 

 residual material; B= the percentage of the same constituent in the fresh 

 rock, and (7=. the quotient obtained by dividing the percentage amount of 

 alumina (or iron sesquioxide, whichever is taken as a constant factor) of 

 the residual material by that in the ±resh rock, the final quotient being 

 multiplied by 100. x then equals the percentage of the original constituent 

 saved, in the residue, and y the percentage of the same constituent lost. 



