﻿G. P. Merrill — " Pliocene Sandstones" 



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Art. XXI. — Notes on the Composition of Certain " Pliocene 

 Sandstones " from Montana and Idaho ; by George P. 

 Merrill. 



A year or more ago, while attempting to classify and 

 arrange the rocks of the collections made in Montana and 

 Idaho in 1871 by Dr. A. C. Peale of the Hayden Survey, my 

 attention was called to a series of fragmental rocks from Little 

 Sage Creek, Beaver Head Cafion, Devil's Pathway, Marsh 

 Creek Valley and Port Neuf Canon, which were labeled as 

 "Pliocene" sandstones. A single glance showed me that 

 these could not be ordinary sandstones, but that they bore a 

 striking resemblance to compacted volcanic dast and sand. 

 A glance at a portion of the dust rubbed from the specimen 

 with the thumb and finger and placed under the microscope 

 was sufficient to confirm my first conclusions, and to show that 

 the stones consisted very largely of minute flakes of pumiceous 

 glass sufficiently compacted to be readily broken out into hand 

 specimens, but yet so friable that in most cases they could be 

 easily disintegrated by the thumb and fingers. On mentioning 

 the fact to Dr. Peale, this gentleman expressed great interest in 

 the matter, and requested that I make a farther examination 

 and put my results upon record for purpose of future reference. 

 This I have accordingly done below. 



Little Sage Creek, Montana. — Three varieties of the sandstone 

 were found from this localitj^ one (35890a)* a fine-grained, 

 light gray and rather friable 

 rock, quite homogeneous, and 

 with a sharp gritty feel, agreeing 

 closely in appearance with a 

 pumiceous clastic rock from Cit- 

 adel Cliff, East Fork of Holmes 

 Creek, collected by the 40th par- 

 allel survey. Portions of the 

 dust rubbed off and placed under 

 the microscope showed it to con- 

 sist wholly of a very light smoky 

 brown, completely amorphous, 

 glass, with the sharp angles and 

 peculiar curved sides so char- 

 acteristic of pumiceous glasses when in a finely divided state 

 (fig. l).f Between crossed nicols there is seen rarely a polariz- 



* The numbers given refer to the catalogue number of the specimen in the 

 Museum collectiou. 



f Other figures showing the appearance of volcanic dust as seen under the 

 microscope are given in the following papers: On Yolcanic Ashes and Cosmic 

 Dust, Nature, April 17th, 1884; Yolcanic Sand which fell at TJnalashka, October 

 20th, 1883, Science. May 30th. 1834: and on Volcanic Dust from Southeastern 

 Nebraska, Proc. Nat. Mus., vol. viii, 1885, p. 99. 



Am. Jour. Sci— Third Series, Vol. XXXII, No. 189.— September, 1386. 

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