30 HENRY S. WASHINGTON 



figures for water which is driven off at iio°, and which cannot be due 

 only to the moisture of the sample, indicates the presence of a zeolitic 

 cement, the amount of which must be very large in the yellow tuffs, in 

 which calcite also is present, and which serves to explain their more 

 coherent character. 



While the processes involved in weathering are highly complex 

 and involve the abstraction of certain constituents, and to a less 

 extent the addition of others, and in varying amounts for each,' so 

 that the calculation of the figures shown by the analysis of weathered 

 rocks to a water- and carbon-dioxide-free basis cannot usually express 

 the original composition of the mass, yet the figures shown in the 

 last two columns, which have been calculated on this basis, are not 

 without interest. In spite of the higher alumina and ferric oxide, 

 and lower ferrous oxide and magnesia, which they show, they indicate 

 that the tuffs are derived from basalts which did not originally differ 

 chemically very materially from the later lavas, especially those of 

 the Monte Ponente type. The scarcely appreciable change in the 

 figures for the alkalies, however, those for soda being even slightly 

 higher in the tuffs than in the lavas, would indicate that the original 

 magma of the tuffs was distinctly higher in these, especially in soda, 

 and that nephelite would probably have been present in considerable 

 amount had they formed compact lavas. The somewhat higher soda 

 in the blocks of lava from the tuffs (page 23), over those of the later 

 lava flows, leading to the presence of nephelite in the norm, and prob- 

 ably in the mode, of these rocks", also points the same way. 



In general, however, regarding the tuffs as weathered representa- 

 tives of original basaltic ones, the processes of alteration seem to have 

 been normal, as explained by Roth^ and Merrill, ^ the latter especially 

 pointing out the usually more rapid loss in magnesia than in hme. 



Although the figures shown in la and 11a above are open to the 

 criticism that they do not adequately represent the original magmas, 

 yet it is instructive to examine the norms which they yield, which are 

 given here, the ratios being omitted. 



1 Cf. G. P. Merrill, Rocks, Rock-weathering, and Soils, New York and London, 

 1897, PP- 173-94- 



2 J. Roth, Allgemeine und chemische Geologic, Vol. Ill, 1893, p. 251. 



3 G. P. Merrill, op. cit., pp. 218-24, 239. 



