522 IOWA ACADEMY OF SCIENCE Vol. XXV, 1918 



elusions he has arrived at after at least a diligent study of the 

 geologrv' of southwestern Iowa. 



In the past conflations of strat-a have almost exclusively been 

 founded on the different limestones, to the general neglect ot 

 the coal seams and black shales ; these latter are much more con- 

 stant and certain horizon markers than the former. Although the 

 limestones of the Missouri stage are fairly constant they are 

 nevertheless given to coalescing with each other and feathering 

 out; the gradual introduction of additional layers to a ledge may 

 change its appearance within a few miles and cause it to be er- 

 roneously correlated. Also the splitting of a thick limestone by 

 a thin shale parting in one locality and the increase in the thick- 

 ness of the shale in another not far distant may divide the ledge 

 by many feet, and so give the appearance of two separate ledges 

 of limestone which seem to belong in an altogether different 

 place in the general section. The paleontological method does 

 not much help the situation as there is no abrupt change in the 

 fauna throughout the whole succession of strata. The contin- 

 ued studies of the writer have so extended the range of the bra- 

 chiopod element as to show that different species range through 

 the whole section. The pelecypods are of more value in corre- 

 lations, especially as the Tarkio is an important Myalina horizon, 

 and in the shales immediately al)ove the Tarkio there are many 

 small pelecypods of dift'icult determination. Also the main lime- 

 stone at Stennett has an exceedingly abundant echinoid fauna. 

 The corals an(jl gastropods are found sporadically througli the 

 whole section. It is thought the bryozoa may be of the utmost 

 value in correlations and that in the future all precise paleonto- 

 logical correlation must be done by the bryozoa. However, their 

 study is most difficult and repellant to the student and practi- 

 cally nothing has been done in that line in Iowa. IMany years 

 ago the late Doctor Calvin in conversation with the writer stated 

 that the fauna of the Missouri stage was found in colonies, and 

 the different species in each colony were very similar. The cor- 

 relation of the strata in southwestern Iowa must mostly be done 

 by the stratigraphioal method although paleontology^ affords a 

 useful check. In a geological field where outcrops will average 

 twenty miles apart correlations should be made with the great- 

 est .caution. Especially would the writer deprecate attempts to 

 correlate the strata in Iowa with those of localities in other 

 states several hundred miles distant. 



