208 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 3902. [bull. 213. 
Princeton there is a very persistent layer of calcareous sandstone, 
varying- from 1 to 12 feet in thickness. 
The St. Louis limestone underlying the Princeton limestone has a 
thickness of about 500 feet. Its basal portion is also oolitic, but of a 
darker color than the Princeton oolites. The remainder consists of 
dark-gray, highly siliceous limestone, the silica of which, on the 
weathering and decomposition of the limestone, to which it is more 
readily subject than the other limestones, is concentrated into nodular 
masses of flinty chert varying from 2 to 8 inches in thickness. These 
rounded lumps often occur in great abundance and are highly charac- 
teristic of the formation. Decomposition of t^he St. Louis limestone 
is always deep, sometimes extending to a depth of 50 feet beneath the 
surface, so that the limestone itself is rarely seen except along rapidly 
eroding si reams. ( )wing to complex faulting the area! distribution of 
these formations is very irregular and patchy. 
Beneath the St. Louis limestone there is an even more' siliceous and 
earthy limestone, representing the Tullahoma formation and Port 
Payne chert of the south, the Keokuk and Burlington limestones of 
western Illinois, and the Boone chert of Missouri and Arkansas. This 
horizon holds mosl of the zinc and lead deposits of the Joplin district 
and some of the deposits found in northern Arkansas. Whether it is 
ore bearing in this district or not can only be determined by sinking 
on the veins to its horizon. 
Structure. — The most marked structural feature of the district is an 
extensive series of fractures, nearly all of which are accompanied by 
more or less faulting. All available evidence tends to the conclusion 
that vein deposits of some kind occur in all the fractures where either 
one oi' both walls are limestone, excepting where the fractures are 
occupied by peridotite dikes. These usually are accompanied by 
only a slight displacement of the strata, and, with a single known, 
but very notable, exception, are not associated with valuable minerals. 
It is a fact that nearly all the mines of the district whose value 
has been proved by development, and nearly all the promising pros- 
pects, have either the St. Louis or the Princeton limestone on one or 
both sides of the fracture. As to the few exceptions where a promis- 
ing prospect occurs in a Chester area, in every case known to me one 
of the limestone beds of that group of rocks forms either the hanging 
or the foot wall of the fissure. We have met with several cases in 
the district that might appear to be exceptions to this rule, notably 
the Clements mine on the Crittenden Springs property, and the east- 
ernmost shaft of the Tabb mines. Critically examined, however, the 
exceptions prove to be more apparent than real, since in the first of 
these cases one of the walls of the adjacent main fault is the Prince- 
ton limestone, and in the other the St. Louis limestone, the openings in 
question being driven in fissures running parallel with and sudsidiary 
to the main faults. These subsidiary fissures were probably formed 
by large slices of country rock breaking away from the hanging wall, 
