THE NONMEf ALLIC MINERALS. 



471 



of the material is due to the thin partitions of glass composing the 

 walls between these vesicles. Any variety of volcanic rock, flowing 

 out upon the surface of the ground, is likely to assume the vesicular 

 condition known as pumiceous, but only certain acid varieties known 

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

 a desirable pumice, and in this rock only in exceptional circumstances. 

 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. 6078T, 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 bolted 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 farther west, have been found extensive beds of a fine. 

 white powder, which was first shown by the present writer 1 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 yeyserite. 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, 00920. 

 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, 2 and (II) a pumice from 

 Capo di Costagna, Lipari Islands: 



1 See On Deposits of Volcanic Dust in Southwestern Nebraska (Proceedings U. S. 

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

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

 1886, p. 199). 



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



