70 KEOKUK GROUP. 



period did the harder parts of Crinoids so completely form the limestone, and 

 hence it is pre-eminently the age of Crinoids. As the Oraptolida reached the 

 height of development in the Quebec or Upper Taconic, the Orihoceratidce in the 

 Black River, and the Cystidea in the Niagara; so did the Crinoidea iu the Burling- 

 ton. The bed of the ocean was covered with a dense growth of Crinoids, one gen- 

 eration after another, while the superincumbent water swarmed with fish and in- 

 vertebrate life. About 400 species of Crinoids, or one-fourth of all known, are 

 from this Group. Among those having the greater distribution and being most 

 characteristic are Dorycrinus missouriensis, D. parvus, J>. unicornis, Batocrinus 

 christyi, B. pyriforrnis, B. rotundus, Actinocrinw proboscidialis, Platycrinus planus, 

 Amphoracrinus diver gens, Belemnocrinus typus, Strotocrinus regalis, Steganoerimis con- 

 cinnus, and Physetocrinus ventricosus. 



CHAPTER XXVIII. 



KEOKUK GROUP. 



$ 147. This Group was named from Keokuk, Iowa, where it was extensively 

 quarried, and known as the Keokuk limestone, before it was known as a geological 

 subdivision. It was first defined by Owen in 1852, and afterward by Hall in 1858. 

 As defined by Hall, it consisted of fifty feet of fossiliferous limestone capped by 40 

 feet of shale, abounding in geodes of quartz, called the geode bed. Others refer 

 the chert layers, which separate it from the Burlington, to this Group. It rapidly 

 thins out to the north, but maintains its thickness southerly to the mouth of the 

 Illinois River, and appears in the south-western part of Missouri, with a thickness 

 of 200 feet, where it is a lead-bearing rock. It crops out in Indiana, 40 or 50 

 miles north-west of Crawfordsville, and extends southerly, crossing into Kentucky 

 a short distance below New Albany. The thickness does not much exceed 100 

 feet. It is celebrated at Crawfordsville for the abundance and perfection of the 

 Crinoids; entire specimens — roots, column, head, arms, and pinnules — have been col- 

 lected. It is well displayed in Southern Kentucky, at King's Mountain tunnel, 

 and in Tennessee, where the thickness is 200 feet. It occurs in Richland County, 

 Ohio, and at other places on the western border of the Appalachian coal basin, but 

 has not been described on the eastern border. It has been identified at numerous 

 places in the western mountain ranges. 



£ 148. Ores of lead and zinc occur in South-western Missouri in pockets and 

 fissures associated with limestone and chert, and some of the mines are very rich 

 and have been largely worked. In New Mexico and south of there, in Mexico, 

 silver and lead occur in veins and fissures, some of the mines being very valuable. 

 Some of the fossils having an extensive distribution, and being characteristic, are 

 Dorycrinus mississippiensis, Cyathocrinus multibraehiatus, Barycrinu* hoveyi, For- 

 Ixsiiicrinus wortheni, Platycrinus hemisphericus, Agaricocrinus americanus, A. wortlteni, 

 Actinocrinus hwei, A. pernodosus, Batocrinus biturbinatus, B. indianensis, Gmiias- 

 teroidocrinus tuberosus, Cyathocrinus subtumidus, Palamcis compressus, Amplexus fragi- 

 lis, Productus vittatus, Orthis keokuk, Spirifera keohuk, S. suborbicularix, Platyceras 

 fissurellum, P. equikiterale, and lAthophaga iUinoisensis. 



