Hie Missouri f.vinl tni<1 Zinc Deposits. — liohertson . 243 
large, circular dome-shaped cavity with a small opening in the 
roof. In the center of this amphitheatre was a huge pile of 
debris which had loosened and fallen from the roof and sides. 
The cave had been formed by the solvent action of water and 
at this point had widened out, producing a large cavity, the 
interior of which was filled as described, by fragments of the 
roof and walls which had become detached and fallen in. The 
filling of the annular space thus left by limestone debris and 
the deposition of barite, calcite and galena in the interstices 
would give just such a circle deposit as seen at the C'onlogue 
mine in Miller count}', or the High Point in Morgan county. 
The disseminated deposits of the southeast are formed by 
a metasomatic interchange of minerals, no cavity existing 
prior to their deposit. 
The filling of the cavities is the next step. The gangue, as 
already stated, consists largely' of country rock and of decom- 
position products derived therefrom. It may be readily con- 
ceived that fragments of rock, perhaps less soluble than the 
rest, becoming detached, would lodge in and assist in the fill- 
ing of the cavities. The clays and sands were undoubted!}'- 
the product of the immense surface erosion then in progress, 
and were transported. Some of the clays — sucli as the tallow- 
clay — were probably tleposited chemically. The minerals 
were undoubtedly deposited from solution sometimes by chem- 
ical interchange, and sometimes by evaporation and concen- 
tration of solutions. It now remains to consider the source of 
the mineral solutions. 
Many theories have been proixiunded regarding tlie source 
of the minerals forming ore bodies. So far as these refer to 
the subject under discussion, they may be expressed in three 
comprehensive hypotheses : 1. That the minerals were origi- 
nally deposited in a concentrated condition. '1. That the 
minerals were derived from great depths. 3. That the min- 
erals were widely diffused, but were gathered together and 
deposited by lateral secretion. The first of these theories in- 
sists that the minerals existed in the original oceanic waters 
in a considerable degree of concentratioii, and were deposited 
directly, or, in the case of disseminated deposits, by metaso- 
nuitic action. It is hard for us to conceive, however, how such 
circumstances could have obtained. Besides the fact that in 
