702 NOTES ON THE GEODES OF ILLINOIS. 
from its cavity in the rock, and unless its regular form caught the 
eye, it would be passed without a second glance as wholly unde- 
serving of notice. A mere ball of clay would possess fully as 
much elegance. But pick up that dull, dirty-looking ball, and, if 
it be a good specimen, its extreme lightness attracts attention and 
excites curiosity, and now the impulse is to see what there is in- 
side. A few strokes of the hammer and it breaks in halves, and 
as it falls apart lo! what wonders are presented to us! Who 
would have imagined that so uncouth and rough an exterior con- 
cealed such a splendid interior,—a crystal grotto, which flashes 
and sparkles when the sunlight strikes it as if made of gems of 
the first water. The geodes contain quite a number of different 
minerals which are very variously arranged in different specimens. 
Some of these minerals have been already alluded to, but they 
deserve a somewhat fuller treatment. As all the geodes are sili- 
ceous on the outside, so by far the most common variety is that 
composed wholly, or nearly so, of quartz. The structure of these 
is almost invariably as follows; first, the outer, earthy coating; 
second, a thin layer of white or whitish chalcedony ; third, a layer 
of clear quartz more or less granulated; and fourth, a layer of | 
crystalline quartz, with simple, pyramidal terminations lining 
the inside. The granular layer does not always extend around the 
entire sphere, but often the crystalline quartz rests directly on the 
chalcedony, and in some there is scarcely any or no granulated 
appearance, but it is present in most that have come under my 
notice. The crystals of the inner layer are usually clear and col- 
orless, but many times they will be tinged a more or less deep 
yellow by oxide of iron, and a few crystals have been found lying 
detached in the cavity of the geode, so bright a yellow as to 
cause them to resemble the topaz very closely, and some have 
been cut and sold as topazes. Not infrequently the crystals have 
a slight bluish tinge which, instead of the brilliaht white lustre, 
gives them a soft and liquid hue, as if just ready to melt into 
purest water. Crystals of smoky quartz are occasionally found, 
and I once had the good fortune to break a rather clumsy speci- 
men about three inches by four inches in size, the inner coating of 
which was rose quartz. Instead of a simple layer of erystals, 
either plain or variously convoluted, specimens are not uncommon 
‘in which the crystals, many of them doubly terminated, are piled 
upon each other in all directions, and sometimes these clusters are 
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