520 STEVENSON— THE FORMATION OF COAL BEDS. [>-'ov- i, 



which contains only a trace of calcareous matter. The Spirifers 

 are represented by two species, one of which is in shale as well as 

 limestone and its size is the same in both. Sandstones with marine 

 forms seem to be few in the Appalachian basin and the fossils are 

 rare; but such sandstones are numerous in the Devonian of that 

 basin. Spirifers of the later type attain great size in the coarse 

 Oriskany sandstone where they are associated with huge Disciiicc; 

 while in the later Devonian they are present in the coarser beds of 

 the Chemung, often appearing in great numbers. The athyrids are 

 the same throughout, whether the rock be limestone or shale. A few 

 of the Pennsylvanian brachiopods occur only where the limestone 

 is free from sand or clay, but that proves only that they preferred 

 the cleaner water; it has no bearing on the question of depth, for 

 within a few yards the limestone shades ofif into calcareous shale. 

 Condit's discovery of the Ames fauna near the western shore line, 

 where the rock is sandy, conglomerate and wave marked, shows that 

 the animals existed in shallow water. Butts's observations on the 

 Ordovician fauna are equally definite. The evidence is so clear 

 respecting the Pennsylvanian brachiopods, that one is tempted to 

 believe about all the genera, as Schuchert has suggested respecting 

 the rhynchonellids, that they were originally forms belonging to 

 shallow water, and that the tendency of so many modern forms to 

 prefer deeper water is a modification due to subsidence of coastal 

 areas. As far as the testimony of brachiopods is concerned, there is 

 every reason to suppose that the marine deposits were laid down in 

 shallow water. 



The pelecypods of the Pennsylvanian within the Appalachian 

 basin are allied, for the most part, to families which in recent times 

 have great vertical distribution. Pectens range from 2 to 200 

 fathoms; Limas from 10 to 150; Areas from low water to 200; 

 but aviculoid forms seem to go no deeper than 20 fathoms. At the 

 same time, one must bear in mind that, while many genera have 

 great vertical range, there is, in most of them, a large number of 

 species which are confined to water as shallow as that preferred by 

 Lingiila and Discina. It is certain that most of the Coal Measures 

 pelecypods thrived best where the water carried clay or sand and 

 thrived poorly where pure limestone was deposited. Few pelecypods 



