ART. XXX -ON SOME DARK SHALE RECENTLY DISCOVERED 

 BELOW THE DEVONIAN LIMESTONES, AT INDEPENDENCE, 

 IOWA ; WITH A NOTICE OF ITS FOSSILS AND DESCRIPTION 

 OF NEW SPECIES. 



By S. Calvin, 



Professor of Geology, State Universitij of Iowa. 



The Devonian deposits of Iowa, as now knowu, may be roughly rep- 

 resented by the annexed diagram, in which 1 

 3 indicates the position of a member of the 

 group recently discovered at Independence, 

 2 consisting of dark argillaceous, with some 

 thin beds of impure, concretionary limestone. 

 1 It has been explored to a depth of 20 or 25 

 feet. No. 2 represents all the beds of what 

 have been termed Devonian limestones in Iowa, and is made up largely 

 of limestones, with associated beds of light-colored shales; estimated 

 thickness, 150 feet. No. 3 is a bed of argillaceous shales exposed at and 

 near Eockford, Iowa, and is referred to in this paper as the Eockford 

 Shales. It abounds in fossils, and weathers, on exposure, into a stiff 

 clay, that has been utilized in the manufacture of brick; observed 

 thickness, 70 feet. 



Until quite recently, Nos. 2 and 3 of the above section were supposed 

 to make up the entire thickness of Devonian rocks in Iowa. No. 2 not 

 only varies, as already indicated, in lithological characters, but the 

 grouping of fossils differs widely in different localities, so much so that 

 competent geologists have referred certain exposures — for example, 

 those at Waterloo — to the Corniferous, and others — as at Independence 

 and Waverly — to the Hamilton. Such references of the above-named 

 exposures will be found in the Twenty-third Eeport on the State Cabi- 

 net of New York, pp. 223-226; and in the same article Professors Hall 

 and Whitfield declare the Eockford shales to be the equivalent of the 

 New York Chemung. On the other hand, Dr. C. A. White — Geology of 

 Iowa, 1870, vol. i, p. 187 — is of opinion that all the Devonian strata of 

 Iowa belong to a single epoch. 



Thus matters stood until about a year or so ago, when D. S. Deeriug 

 called attention to the interesting fact that a dark shale had been ex- 

 posed in working out the layers in the bottom of one of the limestone 

 quarries near Independence. The quarrymen- penetrated the shale to a 

 considerable depth in the hope of finding coal. The shale varies some- 

 what lithologically, but where it presents its most characteristic features 

 it is argillaceous, fine-grained, and highly charged with bituminous 



