200 THE CAMBRIAN. [bull. 81. 



the Potsdam sandstone. 1 In a foot note on page 129 it is said tbat 

 since tbe report was written Mr. Meek discovered a trilobite in the third 

 magnesian limestone, which he and Dr. B. F. Shnmard considered 

 identical with one from the Potsdam sandstone of the Northwest. In 

 the tabulation of the strata as determined in Missouri both the third 

 sandstone, and the fourth magnesian limestone are included under Cal- 

 ciferous sandrock. 2 



When speaking of the geological map and the section of the rocks of 

 Missouri, prepared prior to 1861, Prof. G. C. Swallow states that the 

 Potsdam sandstone rests unconformably upon the Azoic stratified 

 slates of Missouri. 3 



The presence of the third sandstone and the fourth magnesian lime- 

 stone is noted by Mr. F. B. Meek in Morgan and Miller Counties. In 

 Miller County the two formations have a thickness of 33 feet and in Mor- 

 gan County the third sandstone varies from 15 to 30 feet. The fourth 

 magnesian limestone is assigned a thickness of 150 feet. 4 



On the map of Madison County, prepared by Messrs. C. J. Norwood 

 and G. C. Broadhead, the geographic distribution of the rocks referred 

 to the Cambrian and pre-Cambrian is delineated. 5 



Below the third magnesian limestone in Madison, St. Francois, and 

 Iron Counties, Prof. G. C. Broadhead found siliceous or gritstone beds 

 with intercalated magnesian limestones and subjacent to these are mar- 

 ble beds, beneath which occur sandstones, conglomerates, and shales. 

 These are numbered one, two, and three, respectively. In No. 3, beneath 

 the third magnesian limestone, Lingulella lamborni, Meek occurs. On 

 the St. Francois Biver, in Madison County, the lower unaltered sand- 

 stones rest directly on the granite. 6 



At a deep well at the St. Louis County Insane Asylum the following 

 section is referred to as beneath the third magnesian limestone : 



11. 98 feet of third sandstone. 



12. 384 feet of fourth magnesian limestone. 



13. 54 feet of Potsdam sandstone. 



14. 245*4 feet, mostly granite, although a portion of the upper 



part may be sandstone. 



15. 40 feet of granite. 7 



A more detailed account of the strata beneath the third magnesian 

 limestone is given on pages 352-357. The fossiliferous bed is separated 



as a "Lingulagrit." In this there were observed Lingulella lamborni, 







'Geology of Missouri. Silurian System. Geol. Surv. [Missouri; 1st anil 2d ann. repts , parti, 

 1855, pp. 128, 129. 



2 Op. cit., pis. between pp. 60 and 61. 



3 Remarks on the Geological Map and Section of the rocks of Missouri. Am. Nat. , vol. 5, 1871, p. 541. 



4 Reports on the geological survey of the State of Missouri, 1855-1871, 1873, pp. 127, 149. 



6 Atlas accompanying Geological Survey of Missouri, Report, including field work of 1873-1874, 

 vol. 1. Jefferson City, 1874. 

 6 Op. cit., p. 31. 

 ? Op. cit., p., 32. 



