Cross and Eakins — New occurrence of Ptilolite. 97 



tion of which fully confirms the earlier analysis. The work of 

 L. V. Pirsson* upon mordenite has, however, fully confirmed 

 the existence of silicates with the ratio between silica and 

 bases which was found in the original ptilolite. 



The new occurrence of ptilolite is in Custer County, Colo- 

 rado, about three miles southeast of the mining town of Silver 

 Cliff. The rock containing the ptilolite occupies but a few 

 square feet of surface on a low ridge of rhyolitic breccia 

 reaching out from the volcanic center of the Rosita Hills, f 

 and we are indebted to Mr. Thomas Charlton of West Cliffe 

 for calling our attention to the peculiar and seemingly insig- 

 nificant occurrence. The rock is a dull green felsitic mass 

 containing many nearly round vesicles most of which are less 

 than 5 mm in diameter while a few are somewhat larger. They 

 occupy about one-third of the rock. The dull green mass 

 proves on microscopical examination to be a completely devitri- 

 fied pitchstone, and it is probable that this vesicular rock is a 

 remnant of the upper portion of a rhyolite flow seen at a lower 

 level near by, though the connection is covered by " wash " 

 and soil. The rock contains few phenocrysts of feldspar, but a 

 iiuidal structure curving about the vesicles is distinct. Devit- 

 rification proceeded from pearlitic cracks. 



The vesicles of the rock have very smooth walls and most 

 of them have a thin crumbling lining of a pale yellowish or 

 white substance. A few cavities -contain a shell of pale bluish 

 glistening quartz crystals and some are entirely filled b} 7 this 

 mineral. More abundant than the quartz is a colorless or pale 

 blue mineral in minute thin tablets which are often grouped in 

 sheaves or bundles. This mineral was identified as barite by 

 chemical analysis. 



Nearly all the vesicles contain a white mineral in extremely 

 minute needles which form a loose, felt-like mass. Usually 

 they fill the cavity and with a curved dissecting needle one can 

 often remove a little white ball of the substance, apparently 

 perfectly pure. The strong resemblance of this mineral to 

 the ptilolite of Green Mountain was evident at first glance. 



Microscopical study of the needles shows that they are trans- 

 parent, colorless, seldom more than -005 mm and often less than 

 •001 mm in thickness. Only when a number of the needles are 

 arranged in parallel position in a bundle do they perceptibly 

 affect polarized light. In such cases they extinguish ± to the 

 length axis, and by the aid of the quartz plate a negative opti- 

 cal character can be made out. 



*On Mordenite. this Journal, vol. xl, p. 232, 1*890. 



f The geology of this region will be described in a forthcoming monograph of 

 the U. S. Geological Survey. A geological sketch of the Rosita Hills, by Whit- 

 man Cross, was published in the Proceedings of the Colorado Scientific Society 

 for 1890. p. 269. 



