364 C. L. Fenton—The Hackberry Stage of 



the Hystrix faunule, with above them a small portion of 

 the Stromatoporella beds. The balance of this fannnle 

 was either eroded away, or it did not attain its fnll 

 development here, for the beds of the Idiostroma zone 

 appear a short distance away and not nmch above the 

 Stromatoporella beds. The Stromatoporella beds are 

 exposed near Portland in Cerro Gordo County, and also 

 some two and one-half miles sonthwest of Nora Springs, 

 in Floyd Connty. At both of these places Stromato- 

 porella and Syringostroma are very abundant. 



On the map (figs. 1, 2), the Spirifer zone is indicated 

 at those points where there are sufficiently large areas 

 of exposure to allow for mapping on the reduced scale 

 here used. The large area southwest of Nora Springs 

 is mapped as being probably Striatula beneath the gla- 

 cial drift and alluvium. The area to the north, mapped 

 as provisional Spirifer, is on the authority of Webster, 

 as is also most of the Striatula zone area west of Mason 

 City. 



The Owen Sub-stage. 



B, III. The Idiostroma Zone. 



The highest portion of the Hackberry stage at the type 

 locality is composed of four to six and one-half feet of 

 buff or buff-brown, and occasionally grayish limestone, 

 heavily bedded, and crowded with a very slender hydro- 

 zoan that seems to belong to the genus Idiostroma. 

 Associated with this is another species of considerably 

 larger size and lesser abundance. There are also 

 Stromatoporella incrustans (H. & W.), 8. solidula 

 (H. & W.) Naticopsis gigantea H. & W., and other gas- 

 tropods. A photograph of these beds at Hackberry 

 Grove is given here, and their contact with the beds of 

 the Spirifer zone below is well shown. Many large 

 blocks of this limestone have fallen from the cliff, and 

 excellent specimens of the Idiostroma have been secured. 



South and west of the main exposure and near the 

 bank of Hackberry Creek there is another small exposure 

 of the Idiostroma limestones. At this station specimens 

 of Aulopora annectens Clarke are to be found in small 

 numbers. In some cases this form seems to have grown 

 on the hardened sea-mud ; in others on the shells of Nati- 

 copsis gigantea. In the washed material composing the 

 bed of Hackberry Creek there are large numbers of frag- 



