NOTES ON THE GEODES OF ILLINOIS. 703 
loose in the cavity, or the latter may be filled with single crystals, 
often so small as to appear like fine sand. The crystals are not 
always clear, but may be covered with a layer of chalcedony 
sometimes so thin as not to modify the form of the terminations, 
sometimes thick enough to make a papillose surface. This coating 
may be either light blue, flesh color, yellowish white, or a bluish 
opal-white, all of them appearing semi-transparent or of an opaque, 
chalk white. Many of this last color have the crystalline termin- 
ations so modified as to appear cubical.’ In a few very beautiful 
specimens that I have seen, a clear white chalcedonic surface was 
sprinkled all over with small, perfectly colorless crystals, looking 
like fine dew drops on a white flower. This coating becomes 
thicker and thicker, until all trace of the crystals is lost, and then 
we have a layer of chalcedony with a perfectly smooth botryoidal 
or mamillary surface, outside of which is a layer of crystalline 
quartz, then, usually, a granulated layer and then the outer chal- 
cedony layer. The inner layer is sometimes very thick, being 
twice the thickness of all the rest. 
The color of the chalcedony is most often a reddish purple, but 
not very seldom we find a greenish yellow or bluish white and 
rarely a chalk white. In rare cases there are two layers of differ- 
ent color, such as pure white over dark brownish purple. In a 
few cases the surface of the chalcedony is sprinkled with crystals 
of pyrites or other substances, but yet the geodes afford a marked 
contrast in this respect to those lined with crystalline quartz, for 
while the former are remarkably free from what might be termed 
foreign substances and never, so far as I have seen, is any consid- 
erable part of the cavity filled with them, in the latter variety 
we find, resting upon the quartz, all the different minerals found 
in the geodes, and often two or three together nearly filling the 
cavity. Besides the isolated crystals of calcite and dolomite 
found resting on the quartz crystals, we have some in which the 
lining is made up entirely of one or the other of these substances. 
Like all the rest these are siliceous on the outside. In the calcite 
geodes the crystals are usually small and of rhombohedral form, 
which is often obscured by their crowding together or piling upon 
each other. In color they differ, some being colorless and trans- 
parent, many white, or yellowish, or flesh color, and some few 
dark purple or chestnut brown, and in one case a layer of dark 
` brown crystals was ornamented with here and there a cluster of 
pure white ones. Two or more forms of erystallization may occur 
