Pirsson — Microscopical Characters of Volcanic Tuffs. 197 



brownish by limonitic pigment, the color of the chalcedony, as 

 well as that of other patches of finely aggregated particles, is 

 so much deeper by transmitted than by reflected light that it is 

 clear this brown color is in large part due to the unequal 

 refraction and internal reflection of light in passing through 

 aggregated fibers and particles, whereby the blue rays are 

 absorbed and the red-orange ones transmitted, as explained by 

 the writer in the case of spherulites.* This effect is seen in 

 many tuffs. The tuff illustrated is not an absolutely pure 

 vitric type in that occasional fragments of quartz, feldspar, 

 augite, hornblende and iron ore occur in it ; it has been 

 analyzed and previously described. f It has about the strength 

 and coherency of a firm chalk. 



Loose Volcanic Dust. — With a series of excessively fine 

 volcanic dust deposits of a loose uncompacted nature which, in 

 some cases at least, have evidently been transported long 

 distances from their original sources if we may judge by the 

 places in which they occur, Oklahoma for example, a study 

 has been made by directly imbedding the particles, without 

 sectioning, in balsam. In these it has been found that while 



Fig. 3. 



Fig. 3. Shapes of glass particles in volcanic dust. 



the various shapes mentioned above occur to a greater or lesser 

 degree, the most prevalent one is that of a thin film of glass of 

 a triangular shape whose sides are curved, forming a spherical 

 triangle. By raising and lowering the objective one may 

 often observe that the film is not flat, but more or less slightly 

 dished, like a wash-glass, thus indicating that it is part of the 

 glass skin or bulb covering a large bubble. These occur with 

 great variety in the detail of the forms, examples of which are 

 shown in a, fig. 3. Some of these are thicker on one side 

 than on the other, giving at times a sphenoidal or axe like 



* Artificial Lava Flow and Its Spherulitic Crvstallization. This Jour. , 

 vol. xxx, 101, 1910. 



t Castle Mountain Mining District, Bull. U. S. G. S., No. 139, p. 127. 



