PITCHING TROUGHS AND ARCHES. 1211 



the deposits are now found; whereas, as has been pointed out, in the 

 Missouri-Kansas district the source of the ore is believed to be the Silurian 

 dolomite and the deposits are in the overlying Carboniferous limestone. 

 The lead and zinc deposits of central and southeastern Missouri are more 

 nearly analogous in their history to the Upper Mississippi Valley district 

 than to the Missouri- Kansas district. 



PITCHING TROUGHS AND ARCHES. 



Another interesting special case of influence of porosity and structure 

 is that where alternately pervious and impervious layers are in a set of 

 pitching folds. The varying porosity may follow from original difference 

 in the porosity of the layers, or it may result from the deformation itself. 

 The more rigid strata may be deformed by fracture, and the less rigid by 

 flowage. The convex sides of the brittle layers are likely to be more 

 fractured, and therefore more porous than the concave sides. For synclines 

 this would place the more porous parts of a stratum at the bottom and for 

 anticlines at the top. 



Where strata are deformed so as to produce a set of similar or nearly 

 similar folds, if no openings are produced at the synclines and anticlines, 

 the layers on the limbs of the folds must be thinned and those on the arches 

 and troughs thickened, or both. But more frequently, in the zone of 

 fracture, instead of the layers on the limbs being thinned and the arches and 

 troughs thickened sufficiently to occupy all the space, openings form on the 

 anticlines and synclines, thus furnishing' trunk channels. Where the folds 

 are large, on account of their being less weight above an arch than above 

 the corresponding syncline, the openings of the anticlines tend to be larg-er; 

 but this does not apply to anticlines and synclines which are at the same 

 level. 



Any combination of porous layers with impervious layers in folds is 

 likely to give trunk channels for underground water in the troughs above 

 impervious strata, and at the crests below impervious strata. When 

 descending waters come into contact with an impervious stratum, they are 

 deflected toward the synclines, and there finding trunk channels, they follow 

 the troughs downward along the pitch. When ascending waters come into 

 contact with an impervious stratum they are deflected toward the anticlines, 

 and there finding trunk channels they follow the arches upward along the 

 pitch. Therefore, ore deposits produced by descending waters are often 



