454 



SCIENCE. 



[N. S. Vol. I. No. 17. 



cemtier, 1893, who said that at first he supposed it was 

 chalk, but had since come to the conclusion that it is 

 something else. "When I wrote to Mr. Ford that I 

 thought it was volcanic glass, proh.ibly derived from 

 some of the now extinct vents along the Rocky Moun- 

 tain front, he expressed some doubt as to this mode of 

 origin, and said : 



' ' ' This specimen was from a solid hill from thirty to 

 forty feet high, composed entirely of this stufi. The 

 point I make is that, on account of its thickness, the 

 crater must have been somewhere very close, and if so, 

 is it not something heretofore unknown in Texas? 

 The exact locality is on Duck creek, in Dickens county, 

 about 50 miles northwest of the Double Mountain. ' 

 ( Dickens county is in northwestern Texas, in the 

 Brazos River drainage. — Author. ) 



"This specimen undoubtedly comes from the post- 

 Cretaceous formations constituting the great Llano 

 Estacado. Perhaps you will remember that in 1886 

 I collected some similar material from near Wray, 

 Colorado, and Hecla, Nebraska, which was described 

 by Prof. MeiTJll of the National Museum, in the 

 American Journal of Science. This Texas material 

 seems very similar to that of the Colorada-Nebraska 

 locality, both in appearance and in geological position, 

 I wish that more was known of the stratigraphy of 

 the Texas beds. The Colorado specimens occur in 

 what is called the White River Tertiary. ' ' 



An examination by the microscope shows 

 that the white material is volcanic glass, in 

 the angular and fluted foirms figured by Mer- 

 rill,* as characteristic of volcanic dust from 

 Furnas county, in southern Nebraska. Dil- 

 ler t also describes and figures similar forms 

 of glass particles from ISTorway, Krakatoa, 

 Truckee River and Breakhart Hill, the lat- 

 ter a hill to the north of Boston, Mass. 

 In the same article he describes volcanic 

 dust from TJnalashka, which fell in October, 

 1883, and discusses volcanic dusts in gen- 

 eral. Professor Diller concludes that ' ' so far 

 as definite observations have been made, they 

 ■warrant the general assertion, that with oc- 

 casional exceptions, which can be readily 

 explained, volanic dust contains a higher 

 percentage of silica than the lava to which 

 it belongs." 



Professor Diller has also described some 



*Proc. U. S. Nat. Mus. 1885, p. 100. 

 t Science, May 30, 1884. 



volcanic material fi-om Knox county, Ne- 

 braska, and from the West Blue River, 

 Seward county, Nebraska,='= and estimated 

 that about 90% was vocanic dust, there" 

 being also numerous rolled quartz grains. 



The description of the material collected 

 by Professor HiU ft'om Wray (B. & L. R. 

 R.), on the south side of the Republican 

 River, occiu's in an interesting article by 

 Professor Merrill, ' On the Composition of 

 Certain Pliocene Sandstones from Montana 

 and Idaho.'! 



Three figures are given showing the shape 

 of the particles of volcanic glass found in 

 the sandstones. In the material fi-om the 

 Devil's Pathway (No. 35893 " ) " there are 

 many disc-like bodies on the glass particles, 

 coloi'less and nearly circular in outline," 

 but the other figures show angular and 

 fluted forms like those above referred to. 

 Merrill gives analyses of three samples of 

 the volcanic dust from Montana and Idalio, 

 and concludes that they are of andesitic or 

 tractytic origin. His analyses include lime 

 and alkali determinations, and the silica • 

 contents range from 67.76% to 68.92%. 



Merrill also states that some volcanic 

 dust from Krakatoa fell on a ship 885 miles 

 from the source of volcanic activity, so that 

 the existence of a layer of volcanic dust at 

 a given point may not indicate the prox- 

 imity of the volcano from which the ma- 

 terial came, but a deposit fortj^ or more feet 

 thick would hardly form at a great distance 

 from the source. 



The volcanic dust obtained by the writer 

 from a layer in the Neocene Lake beds that 

 underlie Mohawk Valley, in Plumas county, 

 California, likewise resembles in the shape 

 of its particles the dusts figured by Diller 

 and Merrill. An analysis of this material 

 by Dr. W. H. Melville showed that it con- 

 tained 70.64% of silica, and it was there- 



■*See article by J. E. Todd, Science, Vol. VII., p. 

 373. 



tAm. Jour. Sei., Vol. XXXII., pp. 199-204. 



