(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 Sandstone"). At the summit, or close
to the summit, of the Upper Ludlow, is a singular stratum only a
few inches thick (varying 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.
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