n8 HISTORICAL PALEONTOLOGY. 



Ludlow, is a singular stratum only a few inches thick (vary- 

 ing from an inch to a foot), which contains numerous remains 

 of crustaceans and fishes, and is well known under the name 

 of the " bone-bed. " Finally, the Upper Ludlow rock graduates 

 invariably into a series of red sandy deposits, which, when of 

 a flaggy character, are known locally as the " Tile-stones. " 

 These beds are probably to be regarded as the highest member 

 of the Upper Silurian; but they are sometimes looked upon as 

 passage-beds into the Old Red Sandstone, or as the base of 

 this formation. It is, in fact, apparently impossible to draw 

 any actual line of demarcation between the Upper Silurian and 

 the overlying deposits of the Devonian or Old Red Sandstone 

 series. Both in Britain and in America the Lower Devonian 

 beds repose with perfect conformity upon the highest Silurian 

 beds, and the two formations appear to pass into one another 

 by gradual and imperceptible transition. 



The Upper Silurian strata of Britain vary from perhaps 

 3000 or 4000 feet in thickness up to 8000 or 10,000 feet. In 

 North America the corresponding series, though also variable, 

 is generally of much smaller thickness, and may be under 1000 

 feet. The general succession of the Upper Silurian deposits 

 of North America is as follows : 



(1) Medina Sandstone. This constitutes the base of the 

 Upper Silurian, and consists of sandy strata, singularly devoid 

 of life, and passing below in some localities into a conglo- 

 merate (" Oneida Conglomerate"), which is stated to contain 

 pebbles derived from the older beds, and which would thus 

 indicate an unconformity between the Upper and Lower 

 Silurian. 



(2) Clinton Group. Above the Medina sandstone are 

 beds of sandstone and shale, sometimes with calcareous bands, 

 which constitute what is known as the " Clinton Group. " The 

 Medina and Clinton groups are undoubtedly the equivalent of 

 the " May Hill Group " of Britain, as shown by the identity of 

 their fossils. 



(3) Niagara Group. This group consists typically of a 

 series of argillaceous beds ("Niagara Shale") capped by 

 limestone ("Niagara Limestone"); and the name of the 

 group is derived from the fact that it is over limestones of this 

 age that the Niagara river is precipitated to form the great 

 Falls. In places the Niagara group is wholly calcareous, 

 and it is continued upwards into a series of marls and sand- 



