HUDSON RIVER GROUP. 45 



characteristic fossils of the Galena occur in this Group. A petroleum spring rises 

 from this Group on the Grand Manitoulin Island, and saline springs at Varennes 

 evolve large volumes of carbureted hydrogen gas. At one of these springs the 

 gas has been collected in a holder, and employed in lighting a house. The black 

 shales of this Group contain variable amounts of combustible matter, and when 

 distilled they give, beside inflammable gases, portions of oily matter, which in the 

 shales of Colliugwood are equal to four or five per cent. 



CHAPTER XI. 



HUDSON RIVER QROUP. 



89. THE Hudson River Group was named from an exposure near Hudson 

 River in New York, and first defined in the geological report by Vanuxem in 

 1842. At the typical locality it consists of shales, shaly sandstones, slates, and 

 thick-bedded grits, stratified and conformable, alternating many times without any 

 regular order of alternation. It was called the Lorraine Shales by Emmons, who 

 mentions, as occurring at one place in New York, that structure called " Cone within 

 Cone," which is so common in the Devonian and later formations. Its maximum 

 thickness in New York is about 800 feet. 



90. The Group is largely exposed in Pennsylvania and other States in the 

 Appalachian System, as far south as Tennessee, and has a thickness in some places 

 of 1,200 feet. In the latter State it has been called the Nashville Group. It is the 

 surface rock of many counties in Kentucky, extending from above Maysville on the 

 Ohio, to near Louisville. In the south-eastern part of Indiana and the south- 

 western part of Ohio, it consists of alternating layers of blue calcareous clay and 

 limestone, and has a thickness of about 800 feet. It has been called in this section the 

 Blue limestone. It occurs in the northern part of Illinois, southern part of Wis- 

 consin, and north-eastern part of Iowa. Its thickness in these States does not exceed 

 240 feet. In the south-eastern part of Missouri its thickness is about 250 feet, and 

 it appears in Texas and New Mexico. It has a wide geographical range in Can- 

 ada, extending from the Island of Anticosti and the eastern border west, by way of 

 the Great Lakes, to the Red River of the north, and again appearing in the mount- 

 ain ranges bordering the Pacific. In the vicinity of Toronto its thickness is about 

 1,100 feet, but it is much thinner in its western extension, and in the region of the 

 Great Lakes rarely exceeds 100 feet. Its greatest thickness in Eastern Canada is 

 about 2,000 feet. 



91. This Group is persistent and of almost universal distribution, except 

 upon the older rocks that were dry land before its deposition. We would expect 

 to find it almost anywhere on the continent by boring through more recent deposits. 

 It is the equivalent, to some extent, of the Caradoc sandstone, or Bala Group, of 

 England and Wales, and is represented in different European exposures. Like the 

 Trenton and all earlier Groups, it is a marine deposit made in water of consider- 

 able depth, not a littoral or shore-line deposit as the Potsdam Group was, though 

 the sandstone occurring in many of the northern exposures was evidently mechanical 

 and derived from land at no great distance to the north. 



