256 C. Keyes — Affinities of the Alexandrian Series. 



ingto two distinct sub-periods and that the latter are separated 

 by a widespread plane of unconformity, there is urgent need 

 of a serial term to specifically designate the early Silurian 

 rocks of the Ozark region. There are strong paleogeographi- 

 cal reasons for distinguishing the earlier transgression from the 

 south from a later invasion from the north — Pike County, 

 Missouri, being a district where there occurred some overlap- 

 ping. The broader or continental relations are well indicated 

 on Schuchert's maps of the distribution of Silurian rocks.* 

 Instead of attempting to further expand terminology unnec- 

 essarily, it seems to be a step in advance to retain if possible a 

 name already in use, modifying its original signification slightly 

 to meet the exigencies occasioned by more recently discovered 

 facts. In this way the interests of geologic nomenclature 

 appear best subserved and the title Alexandrian series becomes 

 perfected as a valid designation. 



Fig. l. 



The Early Silurian, or southern, deposition in the region is 

 thus represented by the Alexandrian series ; Mid-Silurian, or 

 northern, deposition by the Niagaran series. In Iowa the 

 term Niagaran limestone has been in years past applied, as indi- 

 cated by Calvin, f to all of the Silurian section, or to the upper 

 main body, or Goweran series, as suggested by Norton, J of 

 which the Anamosa dolomite is the chief representative. As 

 now understood, § the Niagaran series of that state comprises 

 only the lower portion of the local Silurian section, represented 

 by the Sabula, Colesburg, Hartwick and Monticello dolomites. 



If the invasion of the Silurian sea from the north was 

 gradual and the vertical movement extensive, it is quite likely 

 that the Bowling Green limestone really represents the deposi- 

 tion of Late Silurian time rather than of the Mid-Silurian 

 period ; and that it is to be associated with the Goweran series. | 



The stratigraphic relationships under consideration are best 

 expressed in diagram (fig. 1). 



*Bull. Geol. Soc. America, vol. xx, pis. 63 and 67, 1910. 

 flowa Geol. Surv., vol. xvii, p. 192, 1907. 

 {Iowa Geol. Surv., vol. xi, p. 302, 1901. 

 ' § Proc. Iowa Acad. Sci., vol. xix, p. 149, 1913. 

 || Iowa Geol. Surv., vol. xxii, p. 154, 1913. 



