Chapter 7 of 20 · 3914 words · ~20 min read

Part 7

DEVON, EARLS OF. From the family of De Redvers (De Ripuariis; Riviers), who had been earls of Devon from about 1100, this title passed to Hugh de Courtenay (c. 1275-1340), the representative of a prominent family in the county (see Gibbon's "digression" in chap. lxi. of the _Decline and Fall_, ed. Bury), but was subsequently forfeited by Thomas Courtenay (1432-1462), a Lancastrian who was beheaded after the battle of Towton. It was revived in 1485 in favour of Edward Courtenay (d. 1509), whose son Sir William (d. 1511) married Catherine, daughter of Edward IV. Too great proximity to the throne led to his attainder, but his son Henry (c. 1498-1539) was restored in blood in 1517 as earl of Devon, and in 1525 was created marquess of Exeter; his second wife was a daughter of William Blount, 4th Lord Mountjoy. The title again suffered forfeiture on Henry's execution, but in 1553 it was recreated for his son Edward (1526-1556). At the latter's death it became dormant in the Courtenay family, till in 1831 a claim by a collateral branch was allowed by the House of Lords, and the earldom of Devon was restored to the peerage, still being held by the head of the Courtenays. The earlier earls of Devon were referred to occasionally as earls of Devonshire, but the former variant has prevailed, and the latter is now solely used for the earldom and dukedom held by the Cavendishes (see DEVONSHIRE, EARLS AND DUKES OF, and also the article COURTENAY).

DEVONIAN SYSTEM, in geology, the name applied to series of stratified fossiliferous and igneous rocks that were formed during the Devonian period, that is, in the interval of time between the close of the Silurian period and the beginning of the Carboniferous; it includes the marine Devonian and an estuarine Old Red Sandstone series of strata. The name "Devonian" was introduced in 1829 by Sir R. Murchison and A. Sedgwick to describe the older rocks of Cornwall and Devon which W. Lonsdale had shown, from an examination of the fossils, to be intermediate between the Silurian and Carboniferous. The same two workers also carried on further researches upon the same rocks of the continent, where already several others, F. Roemer, H. E. Beyrich, &c., were endeavouring to elucidate the succession of strata in this portion of the "Transition Series." The labours of these earlier workers, including in addition to those already mentioned, the brothers F. and G. von Sandberger, A. Dumont, J. Gosselet, E. J. A. d'Archiac, E. P. de Verneuil and H. von Dechen, although somewhat modified by later students, formed the foundation upon which the modern classification of the Devonian rocks is based.

[Illustration: Distribution of Devonian Rocks]

_Stratigraphy of the Devonian Facies._

Notwithstanding the fact that it was in Devonshire and Cornwall that the Devonian rocks were first distinguished, it is in central Europe that the succession of strata is most clearly made out, and here, too, their geological position was first indicated by the founders of the system, Sedgwick and Murchison.

_Continental Europe._--Devonian rocks occupy a large area in the centre of Europe, extending from the Ardennes through the south of Belgium across Rhenish Prussia to Darmstadt. They are best known from the picturesque gorges which have been cut through them by the Rhine below Bingen and by the Moselle below Treves. They reappear from under younger formations in Brittany, in the Harz and Thuringia, and are exposed in Franconia, Saxony, Silesia, North Moravia and eastern Galicia. The principal subdivisions of the system in the more typical areas are indicated in Table I.

This threefold subdivision, with a central mass of calcareous strata, is traceable westwards through Belgium (where the Calcaire de Givet represents the _Stringocephalus_ limestone of the Eifel) and eastwards into the Harz. The rocks reappear with local petrographical modifications, but with a remarkable persistence of general palaeontological characters, in Eastern Thuringia, Franconia, Saxony, Silesia, the north of Moravia and East Galicia. Devonian rocks have been detected among the crumpled rocks of the Styrian Alps by means of the evidence of abundant corals, cephalopods, gasteropods, lamellibranchs and other organic remains. Perhaps in other tracts of the Alps, as well as in the Carpathian range, similar shales, limestones and dolomites, though as yet unfossiliferous, but containing ores of silver, lead, mercury, zinc, cobalt and other metals, may be referable to the Devonian system.

In the centre of Europe, therefore, the Devonian rocks consist of a vast thickness of dark-grey sandy and shaly rocks, with occasional seams of limestone, and in particular with one thick central calcareous zone. These rocks are characterized in the lower zones by numerous broad-winged spirifers and by peculiar trilobites (_Phacops_, _Homalonotus_, &c.) which, though generically like those of the Silurian system, are specifically distinct. The central calcareous zone abounds in corals and crinoids as well as in numerous brachiopods. In the highest bands a profusion of coiled cephalopods (_Clymenia_) occurs in some of the limestones, while the shales are crowded with a small but characteristic ostracod crustacean (_Cypridina_). Here and there traces of fishes have been found, more especially in the Eifel, but seldom in such a state of preservation as to warrant their being assigned to any definite place in the zoological scale. Subsequently, however, E. Beyrich has described from Gerolstein in the Eifel an undoubted species of _Pterichthys_, which, as it cannot be certainly identified with any known form, he names _P. Rhenanus_. A _Coccosteus_ has been described by F. A. Roemer from the Harz, and still later one has been cited from Bicken near Herborn by V. Koenen; but, as Beyrich points out, there may be some doubt as to whether the latter is not a _Pterichthys_. A _Ctenacanthus_, seemingly undistinguishable from the _C. Bohemicus_ of Barrande's Étage G, has also been obtained from the Lower Devonian "Nereitenschichten" of Thuringia. The characteristic _Holoptychius nobilissimus_ has been detected in the Psammite de Condroz, which in Belgium forms a characteristic sandy portion of the Upper Devonian rocks. These are interesting facts, as helping to link the Devonian and Old Red Sandstone types together. But they are as yet too few and unsupported to warrant any large deduction as to the correlations between these types.

It is in the north-east of Europe that the Devonian and Old Red Sandstone appear to be united into one system, where the limestones and marine organisms of the one are interstratified with the fish-bearing sandstones and shales of the other. In Russia, as was shown in the great work _Russia and the Ural Mountains_ by Murchison, De Verneuil and Keyserling, rocks intermediate between the Upper Silurian and Carboniferous Limestone formations cover an extent of surface larger than the British Islands. This wide development arises not from the thickness but from the undisturbed horizontal character of the strata. Like the Silurian formations described elsewhere, they remain to this day nearly as flat and unaltered as they were originally laid down. Judged by mere vertical depth, they present but a meagre representative of the massive Devonian greywacke and limestone of Germany, or of the Old Red Sandstone of Britain. Yet vast though the area is over which they form the surface rock, it is probably only a small portion of their total extent; for they are found turned up from under the newer formations along the flank of the Ural chain. It would thus seem that they spread continuously across the whole breadth of Russia in Europe. Though almost everywhere undisturbed, they afford evidence of some terrestrial oscillation between the time of their formation and that of the Silurian rocks on which they rest, for they are found gradually to overlap Upper and Lower Silurian formations.

TABLE I.

+-------------+-------------------+------------------------------+--------------+-----------------+-----------------+ | | | | Brittany and | | | | Stages. | Ardennes. | Rhineland. | Normandy. | Bohemia. | Harz. | / +-------------+-------------------+------------------------------+--------------+-----------------+-----------------+ | | | Limestone of | Cypridina slates. | Slates of | | Cypridina | U | | | Etroeungt. | Pön sandstone (Sauerland). | Rostellec. | | slates. | P | | Famennien | Psammites of | Crumbly limestone (Kramen- | | | Clymenia | P | | (Clymenia | Condroz (sandy | zelkalk) with Clymenia. | | | limestone and | E | | beds). | series). | Neheim slates in Sauerland, | | | limestone of | R | | | Slates of Famenne | and diabases, tuffs, &c., | | | Altenau. | | | | (shaly series). | in Dillmulde, &c. | | | | D / +-------------+-------------------+------------------------------+--------------+-----------------+-----------------+ E \ | | Slates of | Adorf limestone of Waldeck | Limestone of | | Iberg limestone | V | | | Matagne. | and shales with Goniatites | Cop-Choux | | and Winterberg | O | | Frasnien | Limestones, marls | (Eifel and Aix) = | and green | | limestone; | N | |(Intumesce- | and shale of | Budesheimer shales. | slates of | | also Adorf | I | | cens beds). | Frasne, and | Marls, limestone and dolomite| Travuliors. | | limestone and | A | | | red marble of | with Rhynchonella cuboides | | | shales | N | | | Flanders. | (Flinz in part). | | | (Budesheim). | . | | | | Iberg limestone of Dillmulde.| | | | \ +-------------+-------------------+------------------------------+--------------+-----------------+-----------------+ / | | Limestone of | Stringocephalus limestone, |Limestones | H_{2} (of | Stringocephalus | M | | | Givet. | ironstone of Brilon and | of Chalonnes,| Barrande) dark | shales with | I | | Givérien | | Lahnmulde. | Montjean and | plant-bearing | Flaser and | D | |(Stringocep- | | Upper Lenne shales, crinoidal| l'Ecochère. | shales. | Knollenkalk. | D | | halus beds).| | limestone of Eifel, red | | | Wissenbach | L | | | | sandstones of Aix. | | | slates. | E | | | | Tuffs and diabases of Brilon | | H_{1}. | | | | | | and Lahnmulde. | | | | D / | | | Red conglomerate of Aix. | | | | E \ +-------------+-------------------+------------------------------+--------------+-----------------+-----------------+ V | | | Calceola slates | Calceola beds, Wissenbach | Slates of | G_{3} Cephalo- | Calceola beds. | O | | | and limestones | slates, Lower Lenne beds, | Porsguen, | pod limestone. | Nereite slates, | N | | Eifélien | of Couvin. | Güntroder limestone and | greywacke | G_{2} Tentacu- | slates of | I | | (Calceola | Greywacke with | clay slate of Lahnmulde, | of Fret. | lite limestone.| Wieda and | A | | beds). | Spirifer | Dillmulde, Wildungen, | | G_{3} Knollen- | limestones of | N | | | cultrijugatus. | Griefenstein limestone, | | kalk and | Hasselfeld. | . | | | | Ballersbach limestone. | | mottled Mnenian| | \ | | | | | limestone. | | +-------------+-------------------+------------------------------+--------------+-----------------+-----------------+ / | Coblentzien |Greywacke of | Upper Coblentz slates. | Limestones | | Haupt quartzite | L | | | Hierges. | Red sandstone of Eifel, | of Erbray, | | (of Lossen) = | O | | |Shales and conglom-| Coblentz quartzite, lower | Brulon, Viré| | Rammelsberg | W | | | erate of Burnot | Coblentz slates. | and Néhou, | | slates, Schal- | E | | | with quartzite, | Hunsrück and Siegener | greywacke | | lker slates = | R | | | of Bierlé and | greywacke and slates. | of Faou, | | Kahleberg | | | | red slates of | Taunus quartzite and | sandstone | | sandstone. | D | | | Vireux, greywacke | greywacke. | of Gahard. | F-{2} of | Hercynian slates| E / | | of Montigny, | | | Barrande. | and lime- | V \ | | sandstone of Anor.| | | White Konjeprus | stones. | O | +-------------+-------------------+------------------------------+--------------+ Limestone with | | N | | Gédinnien |Slates of St Hubert| Slates of Gédinne. | Slates and | Hercynian | | I | | | and Fooz, slates | | quartzites | fauna. | | A | | | of Mondrepuits, | | of Plou- | | | N | | | arkose of Weis- | | gastel. | | | . | | | mes, conglomerate | | | | | | | | of Fèpin. | | | | | \ +-------------+-------------------+------------------------------+--------------+-----------------+-----------------+

The chief interest of the Russian rocks of this age lies in the fact, first signalized by Murchison and his associates, that they unite within themselves the characters of the Devonian and the Old Red Sandstone types. In some districts they consist largely of limestones, in others of red sandstones and marls. In the former they present molluscs and other marine organisms of known Devonian species; in the latter they afford remains of fishes, some of which are specifically identical with those of the Old Red Sandstone of Scotland. The distribution of these two palaeontological types in Russia is traced by Murchison to the lithological characters of the rocks, and consequent original diversities of physical conditions, rather than to differences of age. Indeed cases occur where in the same band of rock Devonian shells and Old Red Sandstone fishes lie commingled. In the belt of the formation which extends southwards from Archangel and the White Sea, the strata consist of sands and marls, and contain only fish remains. Traced through the Baltic provinces, they are found to pass into red and green marls, clays, thin limestones and sandstones, with beds of gypsum. In some of the calcareous bands such fossils occur as _Orthis striatula_, _Spiriferina prisca_, _Leptaena productoides_, _Spirifer calcaratus_, _Spirorbis omphaloides_ and _Orthoceras subfusiforme_. In the higher beds _Holoptychius_ and other well-known fishes of the Old Red Sandstone occur. Followed still farther to the south, as far as the watershed between Orel and Voronezh, the Devonian rocks lose their red colour and sandy character, and become thin-bedded yellow limestones, and dolomites with soft green and blue marls. Traces of salt deposits are indicated by occasional saline springs. It is evident that the geographical conditions of the Russian area during the Devonian period must have closely resembled those of the Rhine basin and central England during the Triassic period. The Russian Devonian rocks have been classified in Table II. There is an unquestionable passage of the uppermost Devonian rocks of Russia into the base of the Carboniferous system.

TABLE II.

The Lower Devonian of the Harz contains a fauna which is very different from that of the Rhenish region; to this facies the name "Hercynian" has been applied, and the correlation of the strata has been a source of prolonged discussion among continental geologists. A similar fauna appears in Lower Devonian of Bohemia, in Brittany (limestone of Erbray) and in the Urals. The Upper Devonian of the Harz passes up into the Culm.

In the eastern Thuringian Fichtelgebirge the upper division is represented by _Clymenia_ limestone and _Cypridina_ slates with Adorf limestone, diabase and Planschwitzer tuff in the lower part. The middle division has diabases and tuffs at the top with Tentaculite and Nereite shales and limestones below. The upper part of the Lower Devonian, the sandy shale of Steinach, rests unconformably upon Silurian rocks. In the Carnic Alps are coral reef limestones, the equivalents of the Iberg limestone, which attain an enormous thickness; these are underlain by coral limestones with fossils similar to those of the Konjeprus limestone of Bohemia; below these are shales and nodular limestones with goniatites. The Devonian rocks of Poland are sandy in the lower, and more calcareous in the upper parts. They are of interest because while the upper portions agree closely with the Rhenish facies, from the top of the Coblentzien upwards, in the sandy beds near the base Old Red Sandstone fishes (_Coccosteus_, &c.) are found. In France Devonian rocks are found well developed in Brittany, as indicated in the table, also in Normandy and Maine; in the Boulonnais district only the middle and upper divisions are known. In south France in the neighbourhood of Cabrières, about Montpellier and in the Montagne Noire, all three divisions are found in a highly calcareous condition. Devonian rocks are recognized, though frequently much metamorphosed, on both the northern and southern flanks of the Pyrenees; while on the Spanish peninsula they are extensively developed. In Asturias they are no less than 3280 ft. thick, all three divisions and most of the central European subdivisions are present. In general, the Lower Devonian fossils of Spain bear a marked resemblance to those of Brittany.

_Asia._--From the Ural Mountains eastward, Devonian rocks have been traced from point to point right across Asia. In the Altai Mountains they are represented by limestones of Coblentzien age with a fauna possessing Hercynian features. The same features are observed in the Devonian of the Kougnetsk basin, and in Turkestan. Well-developed quartzites with slates and diabases are found south of Yarkand and Khotan. Middle and Upper Devonian strata are widespread in China. Upper Devonian rocks are recorded from Persia, and from the Hindu Kush on the right bank of the Chitral river.

_England._--In England the original Devonian rocks are developed in Devon and Cornwall and west Somerset. In north Devonshire these rocks consist of sandstones, grits and slates, while in south Devon there are, in addition, thick beds of massive limestone, and intercalations of lavas and tuffs. The interpretation of the stratigraphy in this region is a difficult matter, partly on account of the absence of good exposures with fossils, and partly through the disturbed condition of the rocks. The system has been subdivided as shown in Table III.

TABLE III.

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.

TABLE IV.