The Dustfall of March 9, 1918. 



601 



dust when dry. The following minerals have been recog- 

 nized: feldspar, quartz, opal, limonite, hematite, horn- 

 blende calcite, mica, magnetite, apatite, tourmaline, zir- 

 con. There is also some cloud-like material which may 



Fig. 2. 



Fig. 2. Photomicrograph of dust from the dustfall at Madison, 9 March, 

 1918, showing one of the diatoms. Magnified 360 diameters. 



be kaolin. From Eosiwal measurements 1 the proportion 

 of the chief constituents has been estimated to be : 



Feldspar and quartz 65 to 75%. 



Amorphous material, including limonite, hematite, 



kaolin, opal, etc. 20 to 30%. 

 All other constituents ±5%. 



The feldspar fragments are remarkable on account of 

 the fact that they show no alteration whatever; they 

 are as glassy clear as is the quartz. Both the quartz and 

 feldspar are stained by limonite and hematite, and this 

 condition seems to pervade the fragments so thoroughly 



1 A. Eosiwal, Ueber geometrische Gesteinsanalyzen, Verh. k. k. Eeich- 

 sanstalt, Vienna, 1898, p. 143. 



