z1O HISTORICAL PALAZZONTOLOGY. 
“Coniston Flags” and “ Coniston Grits ”) take the same place 
in the north of England. 
(3) The Ludlow Group is the highest member of the Upper 
Silurian, and consists typically of a lower arenaceous and shaly 
series (the ‘‘Lower Ludlow Rock”) a middle calcareous 
member (the “ Aymestry Limestone”), and an upper shaly and 
sandy series (the ‘‘ Upper Ludlow Rock” and “‘ Downton Sand- 
stone”). At the summit, or close to the summit, of the Upper 
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 a 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. 
