1 8 Natural History Bulletin. 



A sentence or two in the fifth volume of the Geological 

 Survey of Illinois, page 223, would seem to have some bearing 

 on the questions under discussion. The authors of the chap- 

 ter on the Geology of Rock Island county, Messrs. Worthen 

 and Shaw, say: "There are also some brown beds near 

 Andalusia that contain numerous Gastcropods and Orthoccra- 

 tites, and a few miles below, these are overlaid by from eight 

 to ten feet of a brown magnesian limestone that contains casts 

 of a large Spirifer like S. Parryamis and Strophoniena 

 demissa. These brown beds are directly overlaid near the 

 mouth of Stonecoal creek by the sandstones and shales of the 

 coal-measures." 



Near Andalusia then it would seem that we have essentially 

 the same geological phenomena as in the region about the mouth 

 of Pine Creek, with this difference, that the sandstone contain- 

 ing casts of Spirtfera parryana {^S.capax, Hall) is represented 

 by a magnesian limestone. A magnesian limestone as all 

 know, does not preserve calcareous structures, and so in the 

 Devonian dolomite near Andalusia, as in the Niagara and 

 other dolomitic limestones of the northwest, the fossil brachio- 

 pods are, for the most part, preserved only as internal casts 

 of the shell. 



The fact that a dolomitic bed near Andalusia passes into a 

 bed of sandstone farther west in the region of Pine Creek, 

 Iowa, would be in perfect accord with what I have already 

 pointed out in the American Geologist for January, 1888, 

 Vol. I, page 30, — namely, that the great dolomitic masses of 

 strata representing the Niagara, Galena and Lower Mag- 

 nesian limestones of Iowa, were formed off shores, and that 

 further seaward, or at least further to the south and west, 

 where they are generally concealed by newer strata, the 

 place of the dolomites was taken by sandstones and shales. ^ 



ous age, the reader is referred to Hall's Geology of Iowa, Vol. I, part i, pp. 

 120-133; White's Geology of Iowa, pp. 22S-9; Geological Survey of Illinois, 

 Vol. V, pp. 228-232. 



I Notes on the Formations passed through in boring the Deep Well at 

 Washington, Iowa, by Professor S. Calvin, American Geologist, Vol. I, p. 28 

 et. seq. Published January, iSSS. 



