WEBSTER — THE ROCKFORD SHALES OF IOWA. I03 



Over this portion of the area occupied by the shales, the deposition 

 seems to have been more rapid than at other localities, and less well 

 adapted to the preservation of the forms imbedded in it ; and, inferring 

 from the rarity of even the casts, the conditions would seem to have 

 been less favorable to the existence of life. 



The blue clay at this locality, as will be observed, occupies the same 

 position relative to the shales as the blue clay at Rockford, hereafter to 

 be spoken of. 



The condition and thickness of the shales in Worth County is diffi- 

 cult, if not impossible, to determine, owing to the unusual depth of the 

 drift deposit which everywhere occupies the surface. It has been re- 

 ported to me, however, from an apparently reliable source, that the 

 shales are well exposed at a locality some miles south-west from North- 

 wood, in Worth County. I have not personally visited this locality. 



That the relation of the shales and their contained fauna to the sub- 

 jacent strata and its fauna may be better understood, I here give a 

 general outline of that portion of the underlying beds which are in- 

 cluded within the area mapped. The shales everywhere occupy the 

 higher position, being overlaid by no rocks older than the Cretaceous. 



The following section, taken at Rockford (in part compiled from 

 Hall's Ceological Survey of Iowa, Vol. I., Part I., page 309), will show 

 the relation which these beds sustain to the underlying strata at thisplace : 



1. Drift, etc 5 feet. 



2. Decomposing argillaceous shaly limestone, containing 



an abundance of beautifully preserved fossils 25 " 



3. Stiff dark blue clay, devoid of fossils 20 " 



4. A gently inclined surface, mostly unexposed, with buft 



and white striped shaly limestone at base 43ft. ioin. 



5. Unexposed 2 feet. 



6. Dark colored hard limestone 2 " 



7. White pure limestone, with shaly structure (usually 



very fine grained and brittle-, and in places made 



up almost entirely of Stromatopord) 5 ft. 6 in. 



8. Hard buff calcareo-silicious sandstone, containing an 



abundance of Streptorhynchus chemungeti^ls and 

 Spirifera disjuncta, which occur in the form of 

 casts, and all lying with a particular portion of the 

 shell upward 3 feet. 



9. Rather soft ash-colored calcareo-silicious sandstone, 



containing casts of fossils* 2 ''■ 



10. Somewhat arenaceous thin-bedded limestone 3 " 



11. Beds not exposed down to the level of Lime Creek . . 3 



Entire elevation : 1 14 ft. 4 in. 



* Three miles south of Rockford, the beds Nos. 8 and 9 are seen to outcrop in the north bank. 



