388 r//^ MINERAL REGION OF SOUTH-WEST 



( 140 feet or more LimestODe; in some places gray and coarsegrained; ia 

 ( others bluish and fine-grained. 



[a) Alluvium. Frequently contains some ore in loose pieces. 



(h) Probably Lower Coal Measures, with occasional occurrences of ore, 



(c and d) Upper layers of Keokuk Group. Also called Archimedes 

 Limestone. (Sub-Carboniferous sj^stem). 



(c) Eepresents the principal ore-bearing strata; (d) the bed rock in 

 which no ore has as yet been discovered. 



We take from the Geological survey of Missouri, 1873-4, by Prof. G. C. 

 Broadhead, the following geological history of the region under consideration : 



I First Period. — Original deposition of the various stratified rocks, 

 I namely: The "Bed Rock," the alternate layers of Limestone 



and Chert, the Silico-Calc'.te, the Slates and Coal, and of the 



Sandstone. 



These several strata, after their deposition, probably remained unaltered 

 for a very long time and became dr}^, hard and dense before the second 

 period began. 



. § r . . 



o ^ Second Period. — Local dolomization of certain strata of Limestone. Dis- 

 ci ^ I turbances and ruptures in the Chert in consequence of the 

 g S j contraction of the Limestone during the metamorphoric action,. 

 r^ ^ Principal deposition of the ores from watery solution. 



« I. 

 This metamorphic action was confined to a part oi the alternate layers of 

 Limestone and Chert, and very limited in its vertical extent — rarely exceed- 

 ing 20 feet. 



The dolomization of the Limestone, and the simultaneous deposition of 

 the ores, began either from horizontal crevices and then extended through 

 the w^hole mass of one stratum of Limestone and was limited by the layers 

 of Chert above and below, or it began from vertical crevices in the Limestone 

 and formed a mass of Dolomized Limestone, with ore extending along the 

 crevice between the Chert layers, and generally from three to ten feet wide. 

 In the first case the "openings" of Granby were formed, in the second the 

 "runs" of Joplin. 



^ ^ I Third Period. — Dissolution and removal of a part of the Limestone from 



a 



H the Silico-calcite and from the alternate layers of Limestone and 



Hi -I Chert. Gradual breaking down of the remaining concretions and 



of the layers of Chert, and of the strata above. Continued deposi- 



^ p tion of ores in diminished measure. 



In this period the immense accumulations of broken Chert were formed, 

 •which in so many places overlie or accompany the ore dejDOsits. The ore 

 (nearly always Galena) was deposited, in many places, in the fissures and 

 little cracks of the broken Chert beds, in sheets between these layers and ir. 



