CENTURY OF IOWA GEOLOGY 433 



adapted from Lawson's scale. Our rocks prove to be really an in- 

 tegral part of these northern masses, a long tongue of which extends 

 from the Great Lake southwestvvard into Iowa and South Dakota. 



The stratigraphic aspect of the Iowa pre-Cambrian rocks is 

 fundamental. At this time the special geologic significance of the 

 terranes of which they are an integral part, lies in the circumstance 

 that they have suddenly acquired world-wide interest on account of 

 the fact that they supply critical data for evaluating the duration 

 of the pre-Cambrian periods. They give us a basis of comparison 

 of the mid-continental section with the Paleozoic successions as we 

 best know them. They enable us to formulate a systematic 

 scheme of pre-Cambrian stratigraphy that is comparable in its 

 variety, its complexity, its detail and extent, with the post-Cambrian 

 standard which has been evolved during the course of the past 

 century. 



That wide interest aroused by the recent discoveries of abund- 

 ant well-preserved organic remains in rocks of undoubted pre- 

 Cambrian, and hence pre-Paleozoic, age is secondary only to the 

 enthusiasm produced a few months ago by the actual location of 

 the f ossilif erous horizons in the general geological column. As defi- 

 nitely determined these oldest fossil-bearing levels are stratigraphi- 

 cally more than two miles beneath all other known horizons yielding 

 traces of life. These revelations are, of course, as important bio- 

 logically as geologically. They materially modify all of our pre- 

 viously held views on the subject. They open up a more inviting 

 field of investigation than awaited the paleontologists of the first 

 half of the last century when they started to unravel the life record 

 preceding Cretacic time. They promise even greater triumphs than 

 when the Paleozoics first revealed their secrets to Murchison, Sedg- 

 wick and Lonsdale. ^^ 



Thus to the bottom of the general geologic column as usually 

 presented in the textbooks of the science, we are inserting a scale 

 of fossiliferous formations the time-span of which equals or even 

 surpasses in duration that covered by the entire Paleozoic succes- 

 sion. 



ORIGIN OP EPIROTIC DEPOSITS 



In commenting upon the potency of the wind as an erosional 

 agent the late W. J. McGee astutely observes that the satisfactory 

 disposal of the rock-waste of the desert by prodigious eolic exporta- 

 tion supplies the missing link to a rational explanation of all those 



3=Proc. Iowa Acad. Sci., Vol. XXIV, pp. 53-60, 1917. 

 28 



