the Upper Devonian of Iowa. 361 



This gives a maximum of about one hundred feet for 

 the whole Hackberry, although it is very doubtful if it 

 attains this thickness at any one given point. The two 

 lower members are ordinarily of gray-blue color, oxidiz- 

 ing to yellowish. 



B, I. The Striatula Zone. 



At the pits of the Mason City Brick and Tile Company, 

 in the southwest part of Mason City in Cerro Gordo 

 County, the clay-shale of the Sheffield, and the shales and 

 indurated rocks of the lowest zone of the Hackberry, the 

 Striatula zone is well exposed, and the unconformity of 

 the Striatula beds on those of the Sheffield Formation 

 is quite well shown. The thickness of the Sheffield is not 

 definitely known, but there are no indications that it is 

 much less than that of the same beds at Rockford. The 

 Striatula zone attains a maximum thickness of about 

 twenty-five feet, with the lower portion consisting largely 

 of heavy ledges of strongly iron-stained and seemingly 

 dolomitic limestone (Section V). The lower portions 

 of these beds, particularly the indurated layers, contain 

 large numbers of "fucoids" apparently of several spe- 

 cies. The most common is a small form of one-fourth to 

 one-half inch diameter, and so far as observed, without 

 branches. It lies along the bedding planes of the rock in 

 large, intermingled, curling masses, and is very charac- 

 teristic of the lowermost ledges. The other prominent 

 form has a diameter of one to two and one-half inches, 

 branches frequently, and is of considerable length. It 

 is found associated with the first form, but farther up in 

 the beds. 



Some three-quarters of a mile to the northwest of the 

 pit noted in Section V are the pits of the Western States 

 Cement Company. Here the total thickness of the 

 Striatula zone is but seven to nine feet; the indurated 

 beds of the various pits operated by the Mason City 

 Brick and Tile Company are represented by a bed not 

 more than four inches in thickness — usually less, but 

 bearing on its surface the characteristic large fucoids of 

 the middle portion of the beds to the south. The dip of 

 these beds is quite strongly to the southwest, but Mr. 

 A. P. Potts, engineer with the Mason City Brick and Tile 

 Company, tells me that the heavy indurated beds have 

 disappeared before one has passed half-way from the 

 pits of that company to those of the cement works. Sec- 

 tion VI shows the relative thickness of the beds at this 



