704 NOTES ON THE GEODES OF ILLINOIS. 
one upon the other in the same specimen, as, for example, a scal- 
enohedral crystal resting upon differently modified rhombobe- 
. drons. Sometimes the cavity of the geode is filled with a mass 
of white or, rarely, flesh colored calcite. Isolated crystals, or 
groups of crystals of pearl spar or dolomite, are of quite common 
occurrence both in the quartz and calcite geodes, but a complete 
layer of this mineral is less common. The crystals are generally 
quite large as compared with the size of the geode. 
According to Prof. Brush (Ill. Geol. Rep., Vol. 1, p. 91), this 
dolomite contains ‘‘a large per cent. of carbonate of iron with the 
carbonates of lime and magnesia.” Although the usual color of 
the dolomite is either a light yellowish brown or silvery gray of 
different shades, it is often stained a rusty brown by oxide of iron. 
The collector will once in a while meet with a geode, probably 
of quite small size, filled with a fine white powder. Prof. Brush 
states this to be a ‘‘ hydrous silicate of alumina.” The only metals 
thus far found in the geodes are iron and zinc. The iron occurs 
in the form of oxide, in small crystals and in powder öne geode 
about one and a half inches in diameter lined with small quartz 
crystals was filled with the powdered oxide. Besides this, long, 
slender, hair-like crystals of pyrites occur as well as those of 
cubical form. These are scattered over chalcedony, quartz crys- 
tals and calcite, and are also imbedded in the calcite. Zinc 
blende is quite common occurring either in crystals, some of them 
quite highly polished, or in a mass in the centre of solid geodes 
when it is associated with calcite. 
In the northern part of Hancock county a few geodes, all that 
I have seen being lined with quartz crystals, contain asphaltum. 
Prof. Brush describes one of these as “apparently more than half 
filled with asphaltum, breaking with a clear conchoidal fracture, 
having a high lustre and jet black color and containing imbedded 
in it detached crystals of quartz” (Ill. Geol. Rep., Vol. 1, p. 92)- 
This was four by three inches in size. Instead of asphaltum 
some are filled with petroleum. Besides those minerals enu- 
merated, Prof. Brush reports finding in a very few cases minute 
crystals of gypsum and in one of arragonite. Water is found in 
some of the geodes which is bitterish to the taste, and contains, 
according to Prof. Brush, a small per cent. of the sulphates of 
lime and magnesia and a slight trace of silica. As the crust of 
the geodes is wholly impervious to water, this must have been in, 
closed when the crust was formed. As Mr. Worthen remarks 
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