THE NONMETALLTd MINERALS. 



471 



of tli(> iniitcriiil is d\w to tlio thin partitions of gla.ss coniposinj*' the 

 Wiill.s )>('tw«'(Mi those v(\sicl(\s. Any variety of volcanic rock, fiowino' 

 out upon th(^ surface of the ground, is likc^ly to assume the vesicuhir 

 condition known as puniiceous, l)ut only certain acid varieties known 

 as liparites seem to possess just the right degree of viscosity to produce 

 a desiral)le i)umice, and in this rock only in exceptional cii'cumstances. 

 Almost the entire commercial supply of pumice is now brought from the 

 Lipari Islands, a group of volcanoes north of Sicily, in the Mediterra- 

 nean Sea. (Specimen No. 60787, U.S.N. M.) The material is usually 

 brought over in bulk and sold in small pieces in the drug and paint 

 shops, or ground and l)()lted to various degrees of fineness and sold like 

 emery and other abrasive materials. (Specimen No. 54155, U.S.N.M.) 

 At times an inferior grade of pumice has been produced from volcanic 

 flows near Lake Merced, in California. In Harlan County, Nebraska, 

 and adjacent portions of Kansas, as well as in many other of the States 

 and Territories fai'ther west, have been found extensive beds of a line, 

 white powder, which was first shown by the present writer^ to be 

 pumiceous dust, drifted an unknown distance by wind currents and 

 finally deposited in the still waters of a lake. Through a mistaken 

 notion regarding its origin this material was first described in Nebraska 

 as (/e]/serite. So far as the writer is aware, these natural pumice 

 powders have thus far been used only locally for polishing purposes 

 and as a cleansing or scouring agent in soap. As the material exists 

 in almost inexhaustible quantities, it would seem that a wider scope of 

 usefulness might yet be discovered. (Specimens Nos. 53074, 60920, 

 37023, U.S.N.M., from Montana, Washington, and Nebraska.) 



The analyses given below show (I) the composition of the pumice 

 dust of Harlan, Orleans County, Nebraska,'' and (II) a pumice from 

 Capo di Costagna, Lipari Islands: 



' See On Deposits of Volcanic Dust in Southwestern Nebraska (Proceedings IT. S. 

 National ^Musmini, VIII, 1885, p. 99), and Notes on the Composition of Certain Plio- 

 cene Sandstones from Montana and Idaho (American Journal of Science, XXXII, 

 188t), p. 199). 



^ Rocks, Rock-weathering, and Soils, p. 350. 



