PRINCIPLES OF STRATIGRAPHIC CORRELATIONS 529 



Examples of transition from a limestone to a sandstone formation 

 without interruption of deposition are rare. The only good one that has 

 come under my observation is that of the Everett limestone and the 

 Saint Peter sandstone in northern Arkansas. Typically developed, the 

 Everett is a fine-grained, nearly pure limestone, locally exceeding 100 

 feet in thickness. Usually the upper few feet contain a considerable 

 amount of quartz in the shape of floating grains, which become more 

 abundant upward and are rounded like those of the overlying Saint 

 Peter. At other places, as directly south of Yellville, a large part of the 

 limestone may be filled with such grains. Where this is so the calcareous 

 part of the rock is often removed by solution, leaving an incoherent mass 

 of quartz sand. The Saint Peter sandstone itself doubtless was origi- 

 nally cemented with lime, which has since been dissolved and carried 

 away by circulating ground waters. Under Cincinnati, where the cor- 

 responding bed gives rise to an artesian flow of sulpho-saline water, it 

 retains much of its original limy binding. 



From these and other ascertained facts it is inferred that deposition 

 continued practically uninterruptedly in parts of northern Arkansas 

 from the beginning of the Everton limestone to the close of the Joachim. 

 The latter, apparentl}'', is the calcareous offshore phase following the 

 northward advance of the Saint Peter beach. As for the Everton stage 

 of the process, it differed chiefly in that the available supply of quartz 

 sand was then much smaller than it became in the Saint Peter stage. 



Trenton deposition in the area to the north and east of Little Falls, 

 New York, offers an excellent example of gradual passage from limestone 

 to shale deposits. Here only the early part of the Trenton age is repre- 

 sented by limestone. This grows more and more argillaceous upward 

 until it passes, so far as observed without interruption of sedimentation, 

 into black shale, which, resembles the Utica very closely, but is still of 

 lower to middle Trenton age. 



Though transitions from limestone to shale occurred often without 

 break in deposition, this condition is nevertheless one that always de- 

 serves close scrutiny. This is true especially when two limestone forma- 

 tions are separated by a thin bed of shale or of irregularly stratified 

 clayey matter. When the concerned beds are fossiliferous the organic 

 remains will usually indicate at once whether the contact between the 

 lower limestone and the shale marks a break in deposition. When they 

 do the limestone floor is always uneven and generally exhibits other clear 

 evidence of preceding emergence and erosion. 



In the case of moderately thick beds of shale following limestone the 



