wherry: clay from volcanic dust 583 



composition between it and halloysite 9 or leverrierite 10 might be 

 traced, but these resemblances are purely accidental. In basic 

 plagioclase feldspars, and, correspondingly, in andesite glass, the 

 ratio A1 2 3 : Si0 2 does not greatly exceed 1 : 2, and this ratio is 

 preserved in the clay. The same ratio happens to hold in the 

 two minerals mentioned, but there the resemblance ceases. 

 Mere numerical similarity does not imply relationship in such 

 cases, where one substance is a mixture of gels and the 

 others definite (even though somewhat variable) minerals. 



The theory above outlined accounts for the peculiar features of 

 this occurrence in an entirely satisfactery manner. It appears, 

 further, that no other mode of formation would explain many 

 of these features. For instance, all clay brought down by rivers 

 and deposited in the sea contains a large amount of fine quartz 

 grains and rarely any feldspar; but the absence of quartz and 

 the presence of feldspar are two of the most characteristic 

 features of this deposit. The sediments brought into the sea by 

 rivers, moreover, tend to be deposited in lenses or wedges, and 

 their beds rarely extend continuously for more than a few hun- 

 dreds or thousands of feet. As opposed to this relation, we find 

 here beds, from 1 inch to 4 feet thick, extending for a distance of 

 at least 50 miles, without marked variation in their thicknesses. 

 Finally, the clayey matter in normal sediments has been for so 

 considerable a period exposed to such varied chemical agents — 

 rain water containing carbon dioxide, river water containing 

 small amounts of salts, and sea water containing larger amounts 

 of these, — that, although it may remain amorphous, it has at- 

 tained a certain degree of stability and definiteness of composi- 

 tion and, since it is no longer particularly porous, after drying 

 out it does not in general take up water again with marked swell- 

 ing; while the clay here described is highly porous and exhibits 

 the latter property prominently. 



The theory of volcanic-dust origin therefore appears to be the 

 only one that satisfactorily accounts for the features shown by 

 this remarkable deposit. 



9 Halloysite being apparently a stable, definite mineral, corresponding to 

 amorphous kaolinite, as shown by E. S. Larsen and the writer (Journ. Wash. 

 Acad. Sci., 7: 178. 1917.) and by A. F. Rogers (Op. cit. 535). 



10 Larsen, E. S., and Wherry, E. T. Journ. Wash. Acad. Sci., 7: 216. 1917. 



