﻿202 G. P. Merrill—" Pliocene Sandstones" 



cavities even in the larger forms. The other, however small 

 the fragments may be, is always pumiceous th rough the pres- 

 ence of innumerable bubbles and tube-like cavities; a slight 

 difference in viscosity of the original magmas having evidently 

 caused one to become coarsely vesicular, which on breaking up 

 through the sudden explosion of the included steam gives rise 

 to fragments each one of which represents a portion of the par- 

 tition between the cavities or vesicles, while the other, through 

 being more finely pumiceous, gives fragments each one of 

 which, however small, still shows its true pumiceous character. 

 These facts are not indicative of any difference in chemical 

 composition, but as one or the other of the forms is usually 

 found to prevail in samples from various localities the matter 

 seems worthy of note. 



All of the above mentioned dusts yielded water when heated 

 in a closed tube, and fused readily, with swelling, before the 

 blowpipe. Samples submitted to Mr. J. E. Whitfield of the 

 Geological Survey for analysis yielded results as follows: 





(35889a) 



(35890a) 



(35893a) 



Ignition 



6-00 



6-50 



5-60 



Water* 



160 



1-12 



3-46 



Fe 2 3 + Al 2 3 



16-22 



18'24f 



17*18 



Si0 2 



68-92 



65-56 



65-76 



CaO 



1-62 



2-58 



2-30 



MgO 



trace. 



0-72 



trace. 



Na 2 



1-56 



2-08 



2-22 



K 2 



4-00 



3-94 



3-14 



99-92 100-74 99-66 



Accepting the apparently well founded conclusions of others % 

 to the effect that such dusts represent the extreme degrees of 

 acidity of the lavas of which they formed a part, we are led to 

 consider these as of andesitic or possibly trachytic derivation. 



Samples, subsequently submitted by Dr. Peale from above 

 Pass and Bridger Creeks near Bozeman, have also been exam- 

 ined. 



The first of these (38448), marked as " Dry Creek Pliocene," 

 is a fine-grained friable aggregate of a light yellowish gray 

 color. Tested with acicl it was found to contain a little lime, 

 but consists chiefly of glass particles very pumiceous and stained 

 by iron oxides. 



* Water given off at 105° C. 



f Mostly FeO. Mr. Whitfield appends the following note : These analyses not 

 checked by duplicates, there being but a small amount of material, therefore the 

 iron is put down as ferric when most of it is ferrous. The ignition, therefore, is 

 not as high as it should be by reason of the oxidation of the FeO to Fe 2 3 , but is 

 partly balanced by entering the iron as Fe 2 3 . 



% See Science, May 30, 1884. 



