HUDSON PERIOD. 219 



he found the combustible portion amounting to 20.96, or about 

 21 pounds to every 100 of the shale ; in that of Dubuque, 11 to 

 16 per cent.; in that of Herkimer co., N.Y. (Utica shale), 12 to 14 

 per cent. 



The springs of Saratoga and Ballston rise from the lower part 

 of the Hudson River shales. 



II. Life. 



The larger part of the species found in the American rocks 

 of the Hudson period are identical with those of the Trenton, 

 — although a considerable number are still peculiar to the period. 

 The Utica shale, like most fine soft shales, contains few fossils, — the 

 conditions of the formation having been such, apparently, as re- 

 sulted in their complete trituration and destruction. The Hudson 

 River beds, while partly as fine as the Utica shale, are often a little 

 coarser in texture, as if formed of the kind of mud that abounds 

 at moderate depths in marine life ; and these somewhat sandy 

 beds, as well as the limestones, are especially fossiliferous. The 

 species in the limestones are often identical with those of the 

 Trenton period, — a limestone period ; while those of the shales are 

 mostly different. The latter are, for the most part, those that live 

 where the bottom is muddy ; while the species whose relics make 

 the limestones require clear waters, and flourish, like the shells 

 of the coral seas, upon the submerged limestone reef which is in 

 process of formation. 



Plants. 



The plants observed are all Sea-weeds, or Fucoids. 



The great amount of carbonaceous material in the shales was 

 probably derived mostly from these plants : it may have partly 

 come from animal life. 



Animals. 



Where the rocks are sandy or shaly, there are few corals or 

 Trilobites; and these. Hudson River beds in New York are, hence, 

 in strong contrast with those of the Trenton period. At the 

 same time, Conchifers are much more numerous. This is the kind 

 of difference that exists now between the species of a muddy 

 bottom and those of a clear, coral-growing sea. 



A striking feature of the shales is the abundance of Graptolites. 

 They occur in the shales of Tennessee and the Upper Mississippi, 

 as well as those of New York. These feathery species must have 



