WEBSTER THE ROCK.FORD SHALES OF IOWA. 109 



It is true that a few of the more common Brachiopoda and corals 

 found in these beds were worn and smoothed by attrition before being 

 imbedded; but if those forms (mostly Lamellibranchiata) which are 

 supposed to have been transported had been subjected to the same 

 action, all traces of organization would have been obliterated. 



Again, there is no evidence that any of these forms were brought in 

 living, as their remains do not occur outside the shales. 



It seems, then, that only one inference can be drawn, and that is, 

 that all of the forms found in the shales were indigenous; that all of 

 the individuals lived, died, and became imbedded where they are found 

 to-day. 



From the general lithological characters of the shales, it would seem 

 that they originated as a shallow shore deposit formed at about the 

 close of the Devonian age in Iowa. The organic life of the old De- 

 vonian sea, in this portion of its area, culminated in these beds, the 

 underlying rocks, for the most part, being singularly devoid of fossil 

 remains. 



