NOTES ON THE GEODES OF ILLINOIS. 699 
along the Illinois side of the Mississippi, reaching from a short 
distance above Niota on the northern border of Hancock County 
southward beyond Warsaw; and indeed the formation in which 
these specimens occur reaches as far south as the mouth of the 
Illinois River, but by far the greater part that have been collected 
were obtained between Niota and Warsaw, where they occur in 
very much the greatest abundance. Very many fine specimens 
have been obtained across the river in Iowa, and the region over 
which they are distributed is probably larger in this state than in 
Illinois. So far as my own observation goes, the geodes do not 
occur in Illinois at any great distance from the river, nearly all the 
best localities being within three or four miles from the banks ; but 
I do not speak with entire confidence in regard to this as nearly all 
my personal investigation has been confined to localities near the 
river and on the Illinois side. For this reason what may be said 
at this time will refer mainly to the region between Niota and 
Warsaw. The geodes from this region are, many of them, very 
beautiful, being often lined with most brilliant crystals of quartz, 
appearing like miniature caves lined with diamonds, or, less bril- 
liant but more exquisite, some are lined with a frost-work of small 
white crystals of the same substance, or with satin crystals of cal- 
cite and pearl spar. They are imbedded in a soft, brittle, argilla- 
ceous shale, which is sometimes a little calcareous, and in a very 
few instances passes into limestone, although the outer coat- 
ing of the geode is always aluminous. This mass of shaly rock 
forms what is called the “Geode bed,” a member of the Keokuk 
group of the Subcarboniferous age. The thickness of this bed 
varies in different places, but is nowhere very great. At Warsaw 
it is well exposed in a railroad cut; and here, according to Mr. 
Worthen, the thickness is forty feet. (Ill. Geol. Rep., Vol. 1. p- 
335.) No trace of fossils has been discovered in the shale, but in 
some places thin layers of limestone are intermingled with it, and 
these afford the characteristic fossils of the Keokuk limestone. 
Whatever the inner coating of these geodes may be, the shell, or 
crust, is always siliceous. The outer surface, however, is usually 
coated with the same clay-like material as that which composes 
the shale in which they occur. Very often there is an outer layer 
of siliceous clay which forms a part of the crust and sends out 
numerous sharp, irregular projections into the next layer which is, 
probably always, chalcedony. It should be remembered that these, 
