176 
iOWA ACADEMY OF SCIEx\^CES. 
the United States.” A short account of the same topaz is also 
given by Mr. A. N. Ailing.* Dana also mentions this locality. f 
The crystals occur in cavities formed in lava. This rock is 
v/hite or grayish in color and has a peculiarly banded structure 
resembling sandstone. It seems to be made up of loose, 
intensely sharp crystals laid doAvn in layers, as though it might 
have been the settlings of volcanic ejectamenta. It carries 
finely disseminated gold and lead and is a typical rhyolite of 
Pliocene- Tertiary age or of the immediately preceding period. 
The lava overlies all the stratified rock and caps the Dugway 
range. It is in a bed nearly 1,000 feet thick, fifteen miles long 
and from two to five miles wide. The natural occurrence of the 
crystals is in cavities in the rock, but the great majority of 
those seen had been separated from the lava and were scat- 
tered over the surface. The whole mountain was apparently 
covered with them and presented a magnificent appearance in 
the bright sunlight, the crystals gleaming like drops of dew in 
the morning sun. Turn where you would the glitter of the 
bright gems met your eyes. In some places they were so thick 
that one could almost scoop them up. But by far the greater 
number were broken and imperfect, the most perfect ones gen- 
erally being the tips broken off along the basal cleavage planes 
from larger crystals. In the course of four or five hours nearly 
a pint of quite perfect specimens was collected with a much 
larger number of imperfect ones. These crystals varied in size 
from tiny ones no larger than a pin-head to those as large as 
one’s thumb. For the most part they are, as Englemann 
described them, “entirely colorless and transparent,” but some 
wine- colored ones were also discovered. These were always 
and only found in cavities in the rock where they were not 
exposed to the light, it being thus conclusively proven that the 
wine-colored ones fade to white under the influence of bright 
sunlight. This change of color has also been noticed in the 
topaz found at Nathrop, Colorado. 
With the topaz of the latter place crystals of garnet, quartz 
and sanidine are associated in varying quantities. 
It has been suggested that these crystals are of secondary 
formation resulting from the action of acid waters on the rock 
since its deposition. But Cross, in the article above mentioned. 
* Am. Jour. Sci., (3), XXXI tl, 146. 1888. 
t A System of Mineralogy, p. 495. 1693. 
