DAVENPORT ACADEMY <>K NATURAL S( I KM ']•>. 



A DESCRIPTION OF THE ROCKFORD SHALES OF 



IOWA. 



BY CLEMENT L. WEBSTER, STATE UNIVERSITY < >K IOWA. 

 (Read he/ore the Academy, April 2Q, iSSy.) 



Of the Devonian rocks of Iowa — and I might, perhaps, venture to 

 say, the United States — there is no division more interesting to the 

 paleontologist than the Rockford shales. The great variety and beau- 

 tiful preservation of the fossil species contained in them, throw much 

 light upon the history of the latter part of the Devonian age, in the 

 northern part of Iowa. 



It is a significant fact that many of the fossil species contained in the 

 shales approach more nearly the European forms than do the forms 

 from the same horizon in other portions of America. Some of the 

 species, however, are peculiar to the shales, not being found, so far as 

 is at present known, in the strata of any other portion of the world ; 

 and the fact that these beds contain some species whose extreme west- 

 ern limit of distribution was heretofore believed to be hundreds of miles 

 to the east, contributes to their interest. 



In this paper I have given, for the most part, only a general descrip- 

 tion of the strata which underlie the shales, in order that their relation 

 to the shales may be understood. 



The area of the shales is represented on the accompanying map. 

 Whether they extend north of the limit indicated on the map, I am 

 unable to say, as the surface is here everywhere so deeply covered by 

 the drift that no exposure of the underlying strata is to be observed ; 

 and during the Quaternary period, they were considerably acted upon 

 by the moving ice — so much so, indeed, that over limited areas they 

 were removed nearly or quite down to the underlying strata. The ac- 

 companying map shows also the distribution of organic remains in the 

 Devonian rocks underlying the shales. 



The lithological characters, as well as the thickness of this formation, 

 are somewhat varied in different portions of its area. 



At Rockford and Hackberry, it attains a thickness of from twenty- 

 five to forty-five feet, and is made up of thin-bedded, friable, very argil- 

 laceous limestones, varying in color from a light buff to a dark brown, 

 which disintegrate very readily, forming a hard, dark yellow or buff- 



