+-----------------------------+-------------------------------+
| North Devon and West | |
| Somerset. | South Devon. |
+-----------------------------+-------------------------------+
/ | Pilton group. Grits, slates | Ashburton slates. |
U | | and thin limestones. | Livaton slates. |
P | | Baggy group. Sandstones | Red and green Entomis slates |
P < | and slates. | (Famennien). |
E | | Pickwell Down group. | Red and grey slates with |
R | | Dark slates and grits. | tuffs. |
. | | Morte slates (?). | Chudleigh goniatite limestone |
\ | | Petherwyn beds (Frasnien). |
M +-----------------------------+-------------------------------+
I / | Ilfracombe slates with | Torquay and Plymouth |
D | | lenticles of limestone. | limestones and Ashprington |
D < | Combe Martin grits and | volcanic series. (Givétien |
L | | slates. | and Eifélien.) |
E | | | Slates and limestones of |
. \ | | Hope's Nose. |
+-----------------------------+-------------------------------+
L / | Hangman grits and slates. | Looe beds (Cornwall). |
O | | Lynton group, grits and | Meadfoot, Cockington and |
W < | calcareous slates. | Warberry series of slates |
E | | Foreland grits and slates. | and greywackes. (Coblentzien |
R | | | and Gédinnien.) |
. \ +-----------------------------+-------------------------------+
The fossil evidence clearly shows the close agreement of the Rhenish
and south Devonshire areas. In north Devonshire the Devonian rocks
pass upward without break into the Culm.
_North America._--In North America the Devonian rocks are extensively
developed; they have been studied most closely in the New York region,
where they are classified according to Table IV.
The classification below is not capable of application over the states
generally and further details are required from many of the regions
where Devonian rocks have been recognized, but everywhere the broad
threefold division seems to obtain. In Maryland the following
arrangement has been adopted--(1) Helderberg = Coeymans; (2) Oriskany;
(3) Romney = Erian; (4) Jennings = Genesee and Portage; (5) Hampshire
= Catskill in part. In the interior the Helderbergian is missing and
the system commences with (1) Oriskany, (2) Onondaga, (3) Hamilton,
(4) Portage (and Genesee), (5) Chemung.
The Helderbergian series is mainly confined to the eastern part of the
continent; there is a northern development in Maine, and in Canada
(Gaspé, New Brunswick, Nova Scotia and Montreal); an Appalachian belt,
and a lower Mississippian region. The series as a whole is mainly
calcareous (2000 ft. in Gaspé), and thins out towards the west. The
fauna has Hercynian affinities. The Oriskany formation consists
largely of coarse sandstones; it is thin in New York, but in Maryland
and Virginia it is several hundred feet thick. It is more widespread
than the underlying Helderbergian. The Lower Devonian appears to be
thick in northern Maine and in Gaspé, New Brunswick and Nova Scotia,
but neither the palaeontology nor the stratigraphy has been completely
worked out.
In the Middle Devonian the thin clastic deposits at the base, Esopus
and Schoharie grits, have not been differentiated west of the
Appalachian region; but the Onondaga limestones are much more
extensive. The Erian series is often described as the Hamilton series
outside the New York district, where the _Marcellus_ shales are
grouped together with the Hamilton shales, and numerous local
subdivisions are included, as in Ohio, Kentucky and Tennessee. The
rocks are mostly shales or slates, but limestones predominate in the
western development. In Pennsylvania the Hamilton series is from 1500
ft. to 5000 ft. thick, but in the more calcareous western extension it
is much thinner. The _Marcellus_ shales are bituminous in places.
The Senecan series is composed of shallow-water deposits; the Tully
limestone, a local bed in New York, thins out in places into a layer
of pyrites which contains a remarkable dwarfed fauna. The bituminous
Genesee shales are thickest in Pennsylvania (300 ft.); 25 ft. on Lake
Erie. The shales and sandstones of the Portage formation reach 1000
ft. to 1400 ft. in western New York. In the Chautauquan series the
Chemung formation is not always clearly separable from the Portage
beds, it is a sandstone and conglomerate formation which reaches its
maximum thickness (8000 ft.) in Pennsylvania, but thins rapidly
towards the west. In the Catskill region the Upper Devonian has an Old
Red facies--red shales and sandstones with a freshwater and brackish
fauna.