G. H. Gordon — Keokuk Beds at Keokuk. 297 



The Calcareous division consists of forty to sixty feet of 

 limestone in layers varying from three inches to four feet, and 

 separated by one to six inches of clay or shale partings. The 

 thicker beds and those furnishing the best building stone 

 occur in the lower portions of the division. One of these (No. 

 4), a very pure white, subcrystalline layer from three to four 

 feet thick, called the White Ledge by the quarry men, supplied 

 the stone for the noted Mormon temple at Nauvoo. Another 

 lower bed, separated from the White Ledge by six feet of 

 shale and rotten limestone, sometimes furnishes an equally 

 good if not better stone for general use. Below this the layers 

 of the transition beds are thin and abound in chert. 



Cherty bands occur at various intervals throughout the 

 division. They have a concretionary structure, and vary in 

 color from a dark drab to white, though usually more or less 

 mottled. They are sometimes quite prominently discolored 

 by included fossil fragments. The limestone layers often 

 become cherty above with an uneven rolling surface. The 

 more important chert masses have the general appearance of a 

 sedimentary deposit. Nodular masses, however, frequently 

 occur in various forms in the limestone beds. In some cases 

 the siliceous material is distributed about some fossil fragment. 

 In others they present the appearance of having taken the 

 place of a cavity in the stone. The surface of contact is often 

 distinct, the chert separating freely from the limestone, but 

 adjacent parts of the latter are more or less altered to a cherty 

 condition. These nodules have an eel-like form with often 

 one or more bends and are not always parallel to the plane of 

 bedding. A section shows a mottled, grayish white chert on 

 the outside distributed in bands about an inner core of granu- 

 lar material made up principally of fossil fragments in a com- 

 minuted condition. 



The chert beds, aggregating forty feet, which immediately 

 underlie this division were included in it in Hall's Report* but 

 subsequently removed to the Burlington by White.f 



The appearance of these bands of chert at varying inter- 

 vals from the Lower Burlington to the St. Louis limestone 

 inclusive has been commented upon by White, though no 

 explanation of their origin has been attempted.:}: Worthen 



* Geological Survey of Iowa, 1858, vol. i, part 1, p. 94. 



f Geological Survey of Iowa, 1870, vol. i, p. 203 (note). 



% The investigations of G. I Hinde, of the British Museum, on various cherts 

 over the world led him to the conclusion that they are of organic origin. " The 

 chert from the Carboniferous limestones of Ireland was all made chiefly from the 

 siliceous spicules of sponges and the sdica of silicified fossils has the same source." 

 — This Journal, III, xxxiv, 405. See also vol. xxxy, 73. Dr. M. C. White, of 

 New Haven, made the earliest observations upon chert of the " Subcarboniferous 

 limestone of Illinois," and the hornstone of the Corniferous and Black River beds. 

 His paper will be found in this Journal (1862) II, xxxiii, 385. Dana also dis- 

 cusses the subject in his Manual on pages 267, 261, and 305. 



