Correspondence. 57 
can not but be fruitful of results, bearing as it does so strongly upon 
climatic conditions, and will probably be useful as a test of the validity 
of several of the hypotheses brought forward to account for the deposi- 
tion. He has already used it vigorously in opposing the hypothesis of 
aqueous deposition, or a flooded condition of the country, which had 
been advocated by several geologirsts as a plausible explanation. There 
are two or more hypotheses of atmospheric deposition, on which the 
study of fossils will no doubt have a bearing. The hypothesis recently 
suggested by Udden (Jour. Geol. Vol. X, pp. 245-51, 1902), that a snow 
field may have existed outside the ice sheet and collected the dust blown 
in from dry and snowless neighboring districts, would suggest an arc- 
tic fauna which may not be born out by the fossils contained in the 
loess. The hypothesis of exceptional aridity in i/ie Mississippi basin 
at the time of the culmination of the lowan ice sheet, and a resulting 
excessive amount of atmospheric dust may also be tested to some extent 
by a study of the molluscan fauna, just as the opposite hypothesis of the 
flooding of the country has been. It is probable, however, that the 
study of fossils alone will be insufficient to fully clear up the "ques- 
tion of the mode or modes of deposition of the loess. 
Ann Arbor, Michigan, December 2, 1903. 
FRANK LEVERETT. 
Notes on Fluorescent Gems. Following is the substance of a 
paper read at a meeting of the section of Geology and Mineralogy of 
the New York Academy of Sciences. Nov. 16, by Mr. W. G. Levison. 
"Fluorescence or the property of reducing the wave-lengths of cer- 
tain luminous rays enhances the beauty of a few colored gems under 
conditions which lessen he effectiveness of others that do not possess 
this property. Garnet, for instance, which is non-fluorescent, loses its 
rich crimson color and becomes dull gray in pure blue light On the 
contrary, most kinds of ruby and ruby spinel, and pink topaz respond 
to light rays above the red on account of their fluorescence, and in 
blue-violet light still display their characteristic tints. The red color 
of the ruby is somewhat developed by the light of the air-gap spark and 
an uncovered Crookes tube. It is intensely excited by the cathode rays. 
Willemite displays a beautiful greenish yellow color not only in ordi- 
nary light rich in the yellow-green rays, but also in light consisting 
chiefly or wholly of the more refrangible colors in which its character- 
istic color would be effaced but for the possession of fluorescence in 
high degree. This mineral is excited furthermore by some of the ultra- 
violet rays and by the Roentgen and Becquerel rays. 
"Other materials which owe desirable tints to fluorescence are. pearl, 
opal, hyalite, chalcedony and kunzite (the new lilac- spodumene). Hid- 
denite, the green spodumene, seems to be non-fluorescent. Impaired by 
fluorescence are triphane, a yellowish-green spodumene, which exhibits 
pink fluorescence in blue light : emerald, which shows crimson fluor- 
escence in the upper part of the spectrum, and diamond, with greenish- 
blue to blue fluorescence excited by several kinds of energy but more 
or less masked in ordinary light. 
