26o 



PRINCIPLES OF STRATIGRAPHY 



day, a rate so high (120 -|- miles per year, compare column 4 of ta- 

 ble) that the observations are probably erroneous. Observations on 

 seepage of water from canals, etc. (Slichter-65 i^i"), have given 

 rates of 0.2074, 0.2500, 0.3000, and 1.0369 miles per year, values 

 closely in accord with those received by experiment. 



Pervious and Impervious Strata. Strata which, from their por- 

 osity, permit a flow of water through them are called pervious, 

 while those not so constituted are impervious. Quartz sandstones, 

 especially those of uniform grain, are usually the most pervious, 

 and shales the most impervious of stratified rocks. Limestones are 

 generally impervious, owing to the filling of their pores by secondary 

 deposit, a change much less characteristic of quartz sandstones, 

 though also found here. Some limestones are, however, porous 

 enough to be good water bearers, such as chalk and partly consoli- 



FiG. 27. Cross-section through South Dakota artesian basin. Elevations 

 above sea-level. Vertical scale much exaggerated over the hori- 

 zontal. (After Slichter.) 



dated shelly limestone (coquina, etc.) ; limestones rendered porous 

 in the belt of weathering by solution ; and limestones rendered 

 porous by dolomitization through replacement of some of their cal- 

 cite by magnesium carbonate. 



The Deeper Zones of Flozv. Pervious strata confined between 

 impervious ones may, from their position, carry surface waters to 

 great depths and constitute a deeper zone of flow. In thus being 

 confined between impervious layers it differs from the surface zone, 

 which is bounded below. by an impervious layer, but above is not 

 confined, but formed merely by the upper limit of ground water. 

 Whereas the subsurface flow follows the slope of the region, that of 

 the deeper zone depends on geologic structure. The deeper zone 

 also draws its supply from great distances and is independent of the 

 local drainage level, while that of the subsurface zone is local. 

 An example of such a pervious bed is the Dakota sandstone of the 

 Front Range and the Great Plains, which is capped by the impervi- 



