FAUNAL RELATIONS 281 



zons in the section could be made. At the time of Hall's rapid recon- 

 naissance, railroad and highway cuttings were unknown, and the oppor- 

 tunities for making exact observations were not so good as they became 

 a generation later. The green Sweetland shales are even now exposed 

 only at a few points, for they lie in this locality at a level where they 

 were subjected to profound subaerial erosion, which took place just 

 prior to the deposition of the Coal Measures. The Sweetland shales are 

 perhaps 40 feet thick at the point of their greatest development in this 

 locality. They are absent in many sections where carboniferous erosion 

 entirely removed them, and even penetrated the Cedar Valley limestone. 

 The horizon of the Spirifer capax beds is only a few feet beneath the 

 bottom of the Sweetland formation. Above the latter is a yellow sand- 

 stone in the basal part of the Coal Measures. In color, in texture, in 

 general appearance, and in manner of weathering, this sandstone is 

 almost identical with the dolomite carrying Spirifer capax. Strange as the 

 statement may appear, it is only after the closest inspection that the 

 two rocks can be distinguished from each other. On the opposite side 

 of the Mississippi river, at the mouth of Stonecoal creek, the two forma- 

 tions lie in conjunction. 



Unless one is able to examine the several lines of juncture between 

 the different beds in the section, he would hesitate but little in inferring 

 that the casts of Spirifer capax were really from some part of the sand- 

 stone above the shales. Hall's mistake was a very natural one. His 

 hasty parallelism of the Muscatine section with that of Burlington has 

 also many points to sustain it, and after all he does not seem to be so 

 far in the wrong as subsequent writers would have us believe. The 

 most recent information goes to show that in his general assertions he 

 came surprisingly close to the truth. If later investigations have not 

 been erroneous, his correlation of the Sweetland beds, as we now call 

 them, with the green shales at Burlington is essentially correct. There 

 is this difference: The horizon of the Sweetland shales probably comes 

 somewhat lower down than the level of the river at Burlington — some- 

 where in the 150 feet that the shales at the latter place are known to 

 extend below the base of the exposed section. 



Regarding the Devonian age of the Spirifer capax beds of Muscatine 

 Calvin * has given a full account. Still later, Uddenf published many 

 additional details in connection with his report on the geological survey 

 of Muscatine county. Worthen and Shaw | were the first, though only 

 incidentally, to call attention to the fact that the Spirifer capax beds 

 were dolomitic. 



*American Geologist, vol. iii, 1889, p. 25. 



flowa Geol. Survey, vol. ix, 1899, p. 289. 



. I Illinois Geol. Survey, vol. v, 1873, p. 223. 



