19I3-] STEVENSON— FORMATION OF COx\L BEDS. 81 



large percentage of insoluble residue. Lesley's'^' study of the 

 elaborate series of analyses, showing composition of the 115 layers 

 of limestone exposed opposite Harrisburg, led him to conclude that 

 in this exposure two types of deposits alternate ; one is of limestone, 

 with 2 to 3 per cent, of carbonate of magnesium and i to 2 per cent, 

 of insoluble matter; the other, a dolomitic limestone, with 26 to 35 

 per cent, of carbonate of magnesium and the insoluble matter is 

 from 7 to 15 per cent. The large percentage of silicate of aluminum 

 is always in the dolomitic beds. The layers analyzed are from a 

 few inches to 8 feet thick, are distinctly separate and the extreme 

 variations of composition are often in direct contact. One who 

 reads carefully the whole of Lesley's discussion is compelled to 

 recognize that the differences are orignal, not secondary, that they 

 are due to conditions in the drainage area, not to change in composi- 

 tion of the water in which they were deposited. 



The replacement described by Strahan may be due to mineral 

 springs as are the flints of Grand'Croix. 



Remains of animals may be regarded as foreign bodies. Cannel 

 often contains abundance of such remains. In such localities, on 

 both sides of the Atlantic, it has been a rich mine for the palaeon- 

 tologist. Marine shells have been found in ordinary coal. The 

 Harlem coal bed, underlying the Ames limestone, has marine forms 

 in its topmost layer at a locality in Ohio as well as at one in West 

 Virginia and Raymond found a marine shell in the Kittanning coal 

 at a locality in Ohio. Remains of higher animals occur in coals of 

 later age. Anker'^^ examined a brown coal of Molasse age in Styria, 

 which so closely resembles black coal that is distinguishable only by 

 its geological position and its occasional woody structure. Bones 

 are present in 3 layers, where they are very numerous, though frag- 

 mentary. A jawbone, retaining the teeth, was recognized as be- 

 longing to Hyena. Bones of mammals occur frequently in modern 

 swamps. 



"J. P. Lesley, Rep. MM, pp. 360, 361. 



" Anker, " The Occurrence of Bones of Animals in a Coal Mine in 

 Styria," Proc. Geol. Soc. London, Vol. I., 1834, p. 467. 



PROC AMER. PHIL. SOC, LII. 2o8 F, PRINTED MAY I3, I9I3. 



