﻿Cross and Eakins — Ptilolite, a new Mineral. 117 



there is still a loss of about *0013 in the quantities which we 

 used, by igniting for five minutes over a blast lamp. This 

 may be done however, as the ignited BeO is readily soluble in 

 hydrochloric acid. 



In closing we wish to express our thanks to Messrs. Geo. J. 

 Brush, L. Stadtmiiller and Geo. F. Kunz for the rare material 

 which they kindly furnished for carrying out this investigation. 



Mineralogical Laboratory, Sheffield Scientific School, May 28th, 1886. 



Art. XL — Communications from the U. S. Geological Survey, 

 Division of the Rocky Mountains. YIII. On Ptilolite, a new 

 Mineral; by Whitman Cross and L. Gk Eakins.* 



The mineral described beyond occurs in cavities of a more or 

 less vesicular augite-andesite which is found in fragments at a 

 certain geological horizon in the conglomerate beds of Green 

 and Table Mountains, Jefferson County, Colorado. The forma- 

 tion to which this conglomerate belongs is of Tertiary age and 

 will be described in the report upon the geology of the Denver 

 coal-basin now in preparation by the U. S. Geological Survey. 

 For the present article it is sufficient to say that the conglom- 

 erate in question is chiefly composed of worn pebbles and 

 bowlders of andesitic rocks, embracing various types. The 

 rock containing the new mineral has been found at several dif- 

 ferent places upon the mountain slopes, at approximately the 

 same horizon in the series, and its fragments are not so worn 

 and rounded as are those of other rocks in the conglomerate. 

 Various structural modifications of this andesite have been 

 found, ranging from the compact massive rock to that which is 

 decidedly vesicular. 



The massive rock is dull ashen-gray in color, exhibiting a 

 few dark augite prisms and tabular feldspar crystals as the only 

 macroscopic constituents. A microscopical examination shows 

 the rock to consist of a network of small plagioclase microlites 

 with irregular augite grains in very small quantity between 

 them. In spots there seems to be a development of quartz in 

 clusters of minute grains. No glassy base was seen, but certain 

 yellowish globulitic masses are apparently devitrification pro- 

 ducts. Porphyritic crystals of augite, biotite and plagioclase 

 are comparatively rare. Chemical analysis of the massive rock 

 gave the following result : 



* Read before the Colorado Scientific Society, May, 1886. 



