HACKBERRY STAGB OF THE UPPER DEVONIAN. 



183 



of course, is by no means so hard and fast as might be inferred from 

 the diagram. The figures refer to the approximate thicknesses in 

 feet. 



Fig. 3. — Faunules of the Striatula Zone. 



The Cerro Gordo Substage. 

 II. B. — The. Striatula Zone. 



The lowest division of the Hackberry, that referred to by Webster 

 in his earlier publications as the "Lower beds" and the "Lower 

 Horizon," is the one to which Webster and I have applied the 

 name Striatula zo.ne.' The most extensive development of this 

 zone is at the pits of the American Brick and Tile Company and 

 the Mason City Brick and Tile Company (owned and operated 

 by the latter) at Mason City, in Cerro Gordo County, where it 

 attains a thickness of at least twenty-five feet. The lower eighteen 

 to twenty feet are composed of more or less indurated, fairly heavy- 

 bedded strata of coarse, gritty texture, strongly iron stained. 

 These beds contain abundant fucoid remains, the most abundant 

 being a small form one-fourth to one-half inch in diameter. It 

 lies along the bedding planes in tangled, curling masses, and is 

 very characteristic of the lowermost ledges. Further up in the 

 zone a larger, branching form, with a diameter sometimes reaching 

 two and one-half inches, predominates. Both are associated to- 

 gether, but there is a distinct predominance of the small species 

 below, and the large one higher up in the zone. The zone is the 

 Fucoid faunule of Fig. 3. 



Above the indurated beds containing fucoids as the principal 

 fossils are six to eight feet of gritty, soft shales and clay-shales 



'American Midland Naturalist, V., p. 214. 



