TRENTON GROUP. 41 



Minnesota to St. Paul and the Falls of St. Anthony. In Wisconsin, Illinois, and 

 Iowa, the Group is separable into two divisions, the lower one being a blue limestone 

 called the Trenton limestone, and the upper a dolomite called the Galena limestone, 

 which is the lead and zinc bearing rock of that region. The thickness of these 

 divisions is variable, but where most persistent, as in South-western Wisconsin, the 

 upper or Galena has a thickness of 250 feet, and the lower or blue limestone 120 

 feet. It thins northward through Minnesota, and the Galena division gradually 

 disappears. The layers of limestone become thin and shaly, with sometimes ferru- 

 ginous oolitic layers. The Galena appears as a lenticular mass or as thinning toward 

 the east and north, and was apparently derived, so far as detrital matter occurs, 

 from western sources, though a great part, like that of other limestones, was a de- 

 posit from the harder parts of animal organisms. Erratics and bowlders found in 

 Northern Dakota indicate the existence of Galena limestone in the mountains of 

 British America. The Galena contains about 86.6 lead and 13.4 sulphur, and oc- 

 curs in fissures and crevices in the limestone, and not in true veins. The ore is 

 supposed to have been precipitated from an aqueous solution. It was called the 

 Galena limestone from the lead or galena, and from its typical exposure at Galena, 

 Illinois. The lead area is about 4,000 square miles, two-thirds of which is in Wis- 

 consin and the rest about equally divided between Illinois and Iowa. 



§ 81. It forms some large surface exposures in Southern Missouri, where out- 

 crops occur 400 feet in thickness. Numerous outcrops occur among the Western 

 mountain ranges and in the Arctic regions, on King William's Island, at North 

 Somerset, Boothia, and other places. It was found by the artesian boring at Louis- 

 ville, Ky., at Columbus, Ohio, and it is expected it will be found by boring at al- 

 most any place upon the continent, save where the rocks of older date are exposed 

 upon the surface. It was not formed upon the margin of an island or continent, but 

 is a regular sea deposit of general distribution where the depth did not exceed 2,000 

 fathoms. The materials are marine, the mass being remains of organic secretions, 

 with little detrital matter. The fauna was abundant, and embraced representatives 

 of nearly all the great subdivisions of invertebrate life that now have an existence 

 in the ocean, and several orders and classes, as the Graptolites, Cystideans, and 

 Trilobites, which have become extinct. The Graptolites and Trilobites were then 

 on the decline, while Crinoids, Cystideans, Brachiopods, Corals, Gasteropods, and 

 Lamellibranchs were on the increase. 



§ 82. Reeeptaculites oweni is peculiar to and characteristic of the Galena di- 

 vision of this Group, and it is usually accompanied with Lingula quadrata, Mur- 

 chisonia major, Fusispira elongata, and other characteristic species. The species most 

 characteristic of the Trenton Group, and which may be relied upon as determin- 

 ing its age wherever they occur, are Orthis tricenaria, found in New York, Canada, 

 Kentucky, Missouri, and Nevada ; Orthis pectinella, found in New York, Canada, 

 and Kentucky; Cyrtolites compressus, found in New York, Canada, Wisconsin, and 

 Minnesota ; Hybocrinus tumidus, H. conicus, Amygdalocystites fiorealis, A. radiatus, 

 Blastoidocrinus carcliaridens, found at Ottawa, Canada, and High Bridge, Kentucky ; 

 Leperditia fabulites and Conularia quadrata, found in New York, Canada, and Ken- 

 tucky ; and Orthis borealis, found in Canada, Wisconsin, Minnesota, and Kentucky. 

 The genus Amygdalocystites has a wide geographical distribution, though a rare fos- 

 sil in every locality, and, so far as known, is confined to this Group. Other char- 



