1885.] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 33 



VoE. Vlil, T^'o. 3. ^¥ashin§:toii, II. C. -2^^144-93,1885. 



of the powder from the light variety came down, which, on examina- 

 tion with the microscope, proved to be nearly all feldspar, with small 

 l)ortions of the groundmass, and included iron-ores and hornblende 

 particles. After standing over night a further i^recipitation was ob- 

 served to have taken i>lace, which, on examination, proved to be very 

 pure feldspar. A sample of this last submitted to Professor Clarke for 

 further examination yielded 55.29 per cent, of silica, which is about the 

 right proportion for labradorite. Owing, however, to the somewhat crude 

 method of separation, I do not feel at all certain that this is the only 

 feldspar present, and it is very probable that with better facilities other 

 results might be obtained. It is poesible that the first portion to come 

 down may have been anorthite, as suggested by Mr. Diller in Science 

 of January 23, 1885. 



With the dark variety of the rock no satisfactory results could be ob- 

 tained, it being found impossible to separate the very impure feldspar 

 from the groundmass with any facilities at my command. 



Especial thanks are due Professor Clarke and Dr. Chatard for the ex- 

 cellent chemical work done by them in this connection. 



National Museum, February 23, 18S5. 



:N'ote.— In Science of May 30, 1884, Mr. J. S. Diller, of the U. S. 

 Geological Survey, reports on a sample of volcanic sand that fell at 

 TJnalashka October 20, 1883, and which was regarded as doubtless a 

 product of the same volcano as the andesite now under consideration. 

 Tiiis sand gave on analysis 52.48 per cent, of silica, and, reasoning 

 from the stand-point that in process of cooling the most basic minerals 

 of a lava will be first to crystallize, and hence on being blown into frag- 

 ments will give rise to a coarser powder than the more siliceous and 

 still glassy portions, he suggests that in the case in hand the fine sil- 

 iceous dust had been separated out by wind currents and carried to a 

 greater distance before deposition. The sand remaining thus failed 

 to give a correct showing regarding the acidity of the rock from which 

 it was derived. 



In Science of January 23, 1885, af'ter learning the results of the silica 

 determination in analysis II as given above, Mr. Diller expresses sur- 

 prise at the seemingly anomalous case in which a lava is as a whole 

 more basic than the sand derived therefrom. From an examination of 

 the Unalashka sand, however, which Mr. Diller has kindly loaned me, 

 I am convinced that it represents the finely divided state of the lighter- 

 colored and more acidic lava, that given in analysis I, and bears no re- 

 lation to the darker and more basic variety. Indeed, a comparison of 

 the groundmass of the two rocks is in itself sufficient to show that the 

 sand examined by' him could only originate from the lighter-colored 

 xock. Mr. Diller's first suggestion therefore still holds good. 

 Proc. N. M. 85 3 



