466 VT. Cross — Alunite and Diaspore 



350 feet and perhaps 400 feet or more. During this elevation 

 occurred extensive erosion of the Cham plain estuary deposits 

 in the river valley and subsequently followed a depression 

 which has amounted to abont 100 feet at New York and which 

 is apparently continuing at the present day. 



As the land rose from its 80 foot depression at New York 

 there seems to have been a brief period of less rapid elevation 

 during which a second series of estuary terraces and alluvial 

 plains were formed which now stand about 25 feet above 

 tide-level. These have been recognized on Staten Island by 

 Dr. N. L. Britton and may be seeu on the Harlem River near 

 Fordham Heights and at various points on the Long Island 

 Sound shore of Westchester County. 



Art. LY. — On Alunite and Diaspore from the Rosita Hills, 

 Colorado ; by Whitman Cross. 



The occurrences to be described in this article lie between 

 the mining towns of Silver Cliff and Rosita, in Custer County, 

 Colorado. They were discovered while studying the geology 

 of this region, under the direction of Mr. S. F. Emmons, pre- 

 liminary to a report which will appear as a monograph of the 

 U. S. Geological Survey. In order that the geological interest 

 attaching to the occurrences may be fully understood a general 

 sketch of the local geology will be given. 



I. Geological Sketch of the Rosita Hills. 



The name Rosita Hills has been applied in the course of this 

 work to a small group of rounded hills on the eastern slope of 

 the great Wet Mountain Valley, which lies between the Sangre 

 de Cristo and Wet Mountain [or Greenhorn] ranges, at a point 

 south of the Grand Canon of the Arkansas River. They 

 cover an area whose dimensions are about five miles north and 

 south, by four east and west, in which are small cones and 

 smooth-sloped ridges, whose absolute elevations vary from 

 8,900 to 9,700 feet above sea-level, while the western and 

 lower base of the hills is at 8,500 feet The Rosita Hills are 

 made up of volcanic rocks, while Archaean schists surround 

 them on all sides and constitute the floor upon which they rest, 

 there being no sedimentary formations in the vicinity, except- 

 ing local tufa beds. Upon the Hayden Geological map of 

 Colorado, Dr. F. M. Endlich being responsible for this portion, 

 the Rosita Hills are included in a much larger area of eruptive 

 rock [" trachoreite," Endlich.], represented as extending along 

 the base of the Wet Mountains, In this connection it may 



