202 



BOCK DISINTEGRATION" AND DECOMPOSITION 



ing in the residual sand. A large part of the ignition in col- 

 nmns I and VIII is therefore to be accredited to carbonic acid, 

 and not to water of hydration. 



CALCXJLA.TED LoSS OP MATERIAL IN MEDFORD DiABASE. 



CONSTITtTBNTS 



Silica (SiOa) . . . 

 Alumina (AlaOg) 

 Ferric oxide (FesOs) 

 Ferrous oxide (FeO) 

 Lime (CaO) . . . 

 Magnesia (MgO) 

 Manganese (MnO) . 

 Potash (K2O) . . 

 Soda (Na20) . . . 

 Phosphoric acid (P2O5) 

 Ignition 



VIII 



1^ 



Rkoaloulated on 

 Basis of 100 



Fresh 

 Diabase 



47.01% 

 20.11 



3.63 



8.83 



7.06 



3.15 



0.77 



2.14 



3.91 



0.68 



100.00% 



Decomposed 

 Diabase 



} 



44.61 % 

 23.24 



12.71 



6.04 

 2.85 

 0.52 

 1.75 

 3.94 

 0.70 

 3.74 



100.00% 





m M 

 o o 



DO ffl 



s ^ 



|!§ 



8.48 

 0.00 



2.42 



1.83 

 0.68 

 0.32 

 0.62 

 0.60 

 0.08 

 0.00 



14.93% 



XI 



o t3 



fl) _ OS 



S ^ to 



03 OS rt 



P-4 Pd V 



81.97% 

 100.00 



81.90 



74.11 

 78.30 

 58.43 

 70.85 

 87.17 

 88.61 

 100.00 



XII 



bo g 



p] *^ en 

 ^ A "^ 

 Q) 03 eH 



18.03% 

 0.00 



18.10 



26.89 

 21.70 

 41.57 

 29.15 

 12.83 

 11.39 

 0.00 



From columns XI and XII it appears that of all the essential 

 constituents, the lime and potash salts have suffered the most, 

 though the iron oxides have been carried away to the amount 

 of 18.10%. Magnesia has also proven very su.sceptible to the 

 solvent action, disappearing to the amount of 21.70% ; and 

 lastly, silica, to the amount of 18.03%. The small original 

 amounts of manganese and phosphoric acid render the results 

 obtained by these calculations of doubtful value, since it is pos- 

 sible they may be due to errors of analysis. 



In this case, as in that of the granite from the District 

 of Columbia, we have to do with only the earlier stages of de- 

 generation, with conditions which are as much in the nature 

 of mechanical disintegration as of chemical decomposition. As 

 before, then, it will be instructive to consider cases in which, in 

 rocks of similar nature, the decomposition has proceeded much 

 farther. For this purpose we will select a diabase from near 



