166 G. P. MERRILL — WEATHERING OF MICACKOUS GNEISS. 



It will be noted at once that even in these five potash plays a compara- 

 tively insignificant part, being highest in phillipsite (6.4 per cent), and 

 averaging for all a little less than 4 per cent. Assuming, then, that a 

 soil contained as high as 10 per cent of zeolitic material all belonging to 

 these five groups and none to the potash-free varieties, even then we 

 would have this combined only some 0.4 per cent of K 2 0. It appears to 

 the writer that these figures are sufficient to throw considerable doubt on 

 the subject of either the presence or the utility of zeolites in soils. It 

 is possible that in nature the process of potash replacement as performed 

 in the laboratory has gone on, and that there may exist in the soil zeolitic 

 compounds richer in potash than are at present known to the mineralo- 

 gist. Such a condition, however, can not be considered probable. 



Reference should be made in this connection to the use of the term 

 zeolite as found in the American literature bearing upon soil fertility. 



It is apparent that the word is, in only too many instances, used almost 

 wholly without regard to its mineralogical significance, and made to in- 

 clude a considerable variety of secondary hydrous minerals. One writer 

 refers to the magnesia in soils as being in zeolitic combinations, and still 

 another includes glauconite under this head. So comprehensive a usage 

 is objectionable, since the word has a fairly well defined mineralogical 

 significance. If the chemists so extend it as to include glauconite, it can 

 be made also to include serpentine, talc, and even kaolin, and the word 

 becomes of so little significance as to be useless. 



Proportional Amounts of soluble Matter in fresh and decomposed 



Rocks. 



There is an abundant opportunity for more work in the way of ascer- 

 taining the proportional amount of soluble matter in rocks and in soils 

 which result from their breaking down. As the writer has pointed out* 

 there may, particularly among basic rocks rich in magnesia, be actually 

 a larger percentage of matter soluble in hydrochloric acid and sodium 

 carbonate solutions in the undecomposed rock than in the soil. This, 

 for the simple reason noted in the paper above referred to, namely, that 

 the decomposition is accompanied by a leaching process, whereby soluble 

 compounds are removed by atmospheric waters. Even in cases where 

 the actual percentage of soluble matter is greatest in the soil, the apparent 

 excess may be due to water of hydration and to the large amount of 

 sesquioxide of iron in the latter, the last, while practically insoluble in 

 meteoric waters so long as there is a free supply of oxygen, being readily 

 soluble in hydrochloric acid. In order to emphasize this point I have 

 tabulated below the analyses of the soluble and insoluble portions of 



*Bull. Geol. Soc. Am., vol. 7, 1896, p. 353. 



