GEOLOGY OF SOUTHWESTERN IOWA 533 



are many chances of error in exact correlations fifty miles apart 

 in a heavy drift covered region. 



Two miles southeast of Clarinda the writer was fortunate to 

 discover an excellent exposure of the cap rock of the Nodaway 

 coal at the site of the old Shambaugh mill, locally known as Pin- 

 hook. A drainage canal has diverted the course of the river 

 some distance to the west. The old river bed is silted up, with 

 brush growing in it. The outcrop is not in as good condition a.' 

 it was at the time of Doctor Calvin's visit when he obtained his 

 section at an old mine drift now completely slumped away. How- 

 ever, the cap rock and roof shale of the coal are well exposed. 

 Doctor Calvin's section is better than any to be obtained at pres- 

 ent and will be given : 



SECTION AT PINHOOK NEAR CLARINDA. 



Feet Inches 



9. Yellow weathered shale 4 



8. Black shale 1 



7. Yellow shale 1 3 



G. Yellowish Impure limestone wihich at the north end 



of the exposure is in two layers, the upper 14 and the 

 lower 18 inches in thickness. The lower bed thins 

 and runs out in a few yards to the south. Average 

 thickness 2 



5. Yellowish shale, present in some parts of the expos- 

 ure and absent in others 6 



4. Black, slaty shale 6 



8. Grayish, fossiliferous, non-laminated shale which dis- 

 appears and re-appears in distances of a few yards. 

 Among the fossil species noted are: Lophophyllum 

 profunduTn, plates and spines of Zeacrinus, Rhomho- 

 pora lepidodendroides, Chonetes granulifer, Productus 

 perteniais, Derbya crassa, represented by numerous 

 small, fragile individuals, Spiriferina kentuckiensis. 

 Ambocoelia planoronvexa, represented by detached 

 valves but very abundant, Athyris subtilita, Strap- 

 arollis catilloides, BeUerophon percarinatus. Belle- 

 rophon oarhonarius, and a small pleurotomaria .... 2 



2. Coal 1 6 



1. Drab shale down to river 8 



Doctor Calvin's faunal list of the roof shales gives the domi- 

 nant fossils of this horizon. The lithology of the cap rock is 

 such that it can be readily recognized. It is an impure limestone, 

 subcrystailine, in many places reddish in color, and breaks with 

 a splintery fracture. This cap rock has been correlated with the 

 Howard formation of Kansas, a name derived from a locality 

 near the southern border of that state. It is inconceivable a 

 thin ledge of limestone less than three feet thick should preserve 

 an unbroken continuity for a distance of two hundred and fifty 



