wherry: clay from volcanic dust 579 



7. Hill slopes one mile northeast of Ardmore, 2 miles from No. 6. 



8. Hill slopes 1? miles southeast o; Provo, South Dakota, about 10 miles north- 

 west of No. 6. 



9. Ledges k mile southeast of Pedro, Wyoming, about 50 miles northwest of 

 No. 8. 



on their standing several hours in contact with the immersion 

 oils. 



By virtue of its porosity the clay is able also to adsorb dyes, 

 especially methylene blue. A moderately strong aqueous solu- 

 tion of this substance is completely decolorized on filtering it 

 through a thin layer of the clay, which becomes deep blue. This 

 behavior, in connection with the absorption of water and accom- 

 panying swelling, points clearly to its amorphous, colloidal 

 nature. The faint, variable, double refraction which it exhibits 

 when examined in oils does not oppose this view, not being nec- 

 essarily indicative of crystallinity. 3 There is every reason to 

 believe, on the contrary, that it is connected with strain in the 

 amorphous, colloidal mass, incident to partial loss of water. 

 The practical disappearance of the double refraction when water 

 is added is the result of the relief of this strain, the normal, 

 amorphous condition being thereupon restored. 



Average samples contain, in addition to the clay particles, a 

 very few grains of feldspar, biotite, and magnetite. Samples 

 collected from different parts of the main bed (or, for that mat- 

 ter, from any of the beds) show, however, a marked downward 

 increase in the amount of such minerals. When a lump of the 

 clay from the lowermost 2 inches of this bed is stirred up in 

 water several times and the cloudy liquid poured off from the 

 solid matter, a very noticeable residue remains. This is found 

 on microscopic examination to contain the following minerals, 

 all in practically fresh, unaltered condition: plagioclase feldspar 

 — mostly andesine, mica — mostly biotite, magnetite, and cal- 

 cite (the last representing a precipitate from the sea water). 



3 That optical anomalies, and especially double refraction in normally iso- 

 tropic minerals, can be produced by strain is well known; and since the prepara- 

 tion of this paper, this point has been emphasised by A. F. Rogers (Journ. Geol., 

 35:519. 1917). 



