Paradoxical as it may appear, this period of geological history has been held as of little account, and has even been by some geologists regarded as scarcely a distinct age, just because it was one of the most striking and important of the whole. The Devonian was an age of change and transition, in both physical and organic existence; and an age which introduced, in the Northern hemisphere at least, more varied conditions of land and water and climate than had previously existed. Hence, over large areas of our continents, its deposits are irregular and locally diverse; and the duration and importance of the period are to be measured rather by the changes and alterations of previous formations, and the ejection of masses of molten rock from beneath, than by a series of fossiliferous deposits. Nevertheless, in some regions in North America and Eastern Europe, the formations of this era are of vast extent and volume, those of North America being estimated at the enormous thickness of 15,000 feet, while they are spread over areas of almost continental breadth.
At the close of the Upper Silurian, the vast continental plateaus of the northern hemisphere were almost wholly submerged. No previous marine limestone spreads more widely than that of the Uppei Silurian, and in no previous period have we much less evidence of the existence of dry land; yet before the end of the period we observe, in a few fragments of land plants scattered here and there in the marine limestones—evidence that islands rose amid the waste of waters. As it is said that the sailors of Columbus saw the first indications of the still unseen Western Continent in drift canes, and fragments of trees floating in mid ocean, so the voyager through the Silurian seas finds his approach to the verdant shores of the Devonian presaged by a few drift plants borne from shores yet below the horizon. The small remains of land in the Upper Silurian were apparently limited to certain clusters of islands in the north-eastern part of America and north-western part of Europe, with perhaps some in the intervening Atlantic On these limited surfaces grew the first land plants certainly known to us—herbs and trees allied to the modern club-mosses, and perhaps forests of trees allied to the pines, though of humbler type; and this wide Upper Silurian sea, with archipelagos of wooded islands, may have continued for a long time. But with the beginning of the Devonian, indications of an unstable condition of the earth’s crust began to develop themselves. New lands were upheaved; great shallow, muddy, and sandy flats were deposited around them the domains of corals and sea-weeds were contracted and on banks, and in shallows and estuaries, there swarmed shoals of fishes of many species, and some of them of most remarkable organization. On the margins of these waters stretched vast swamps, covered with a rank vegetation.
But the period was one of powerful igneous activity. Volcanoes poured out their molten rocks over sea and land, and injected huge dykes of trap into the newly-formed beds. The land was shaken with earthquake throes, and was subject to many upheavals and subsidences. Violent waves desolated the coasts, throwing sand and gravel over the flatk, and tearing up newly-deposited beds; and poisonous exhalations, or sudden changes of level, often proved fatal to immense shoals of fishes. This was the time of the Lower Devonian, and it is marked, both in the old world and the new, by extensive deposits of sandstones and conglomerates.
But the changes going on at the surface were only symptomatic of those occurring beneath. The immense accumulations of Silurian sediment had by this time so overweighted certain portions of the crust, that great quantities of aqueous sediment had been pressed downward into the heated bowels of the earth, and were undergoing, under an enormous weight of superincumbent material, a process of baking and semi-fusion. This process was of course extremely active along the margins of the old Silurian plateaus, and led to great elevation of land, while in the more central parts of the plateaus the oceanic conditions still continued; and in the Middle Devonian, in America at least, one of the most remarkable and interesting coral limestones in the world—the corniferous limestone—was deposited. In process of time, however, these clear waters became shallow, and were invaded by muddy sediments; and in the Upper Devonian the swampy flats and muddy shallows return in full force, and in some degree anticipate the still greater areas of this kind which existed in the succeeding Coal formation.
Such is a brief sketch of the Devonian, or, as it may be better called in America, from the vast development of its beds on the south side of Lake Erie, the Erian formation. In America the marine beds of the Devonian were deposited on the same great continental plateau which supported the seas of the Upper and Lower Silurian, and the beds were thicker towards the east and thinned towards the west, as in the case of the older serios. But in the Devonian there was much, land in the north-east of America; and on the eastern margin of this land, as in Gaspé and New Brunswick, the deposits throughout the whole period were sandstones and shales, without the great coral limestones of the central plateau. Something of the same kind occurred in Europe, where, however, the area of Devonian sea was smaller. There the fossiliferous limestones of the Middle Devonian in Devon, in the Eifel district, in France and in Russia, represent the great corniferous limestone of America; while the sandstones of South Wales, of Ireland, and of Scotland, resemble the local conditions of Gaspé and New Brunswick, and belonged to a similar area in the north-west of Europe, in which shallow water and land conditions prevailed during the whole of the Devonian, and which was perhaps connected with the corresponding region in Eastern America by a North Atlantic archipelago, now submerged. This whole subject is so important to the knowledge of the Devonian, and of geology in general, that I may be pardoned for introducing it here in a tabular form, taking the European series from Etheridge’s excellent and exhaustive paper in the “Journal of the Geological Society.”
DEVONIAN OF ERIAN.
| DIVISIONS. | CENTRAL AREAS. | |||
| Devon. | Rhen. Prussia. | New York. | ||
| Upper |  | Pilton group:— | Clymenia, Cypridina, etc. Shales, limestones, and sandstones. | Chemung and Portage. |
| Middle | | Ilfracombe group:— | Eifel limestone, Calceola shales, etc. | Hamilton shales, and Corniferous or cherty limestone. |
| Lower | | Lynton group:— | Coblentz and Wisseubach shales, Rhenish greywacke, Spinier sandstone. | Schoharie and Caudagalli grits. |
| DIVISIONS. | MARGINAL AREAS. | |||
| Scotland. | Ireland. | Gaspé and New Brunswick. | ||
| Upper | | Yellow and red sandstones. | Yellow and red sandstones, etc. | Red and grey sandstones, grits and shales, and conglomerates of Gaspé and Miapeck. |
| Middle | | Red shales and sandstones, and conglomerates. | Grits and sandstones of Dingle. | Grey and Red sandstones, and grey and dark shales. |
| Lower | | Flagstones, shales and conglomerates. | Glengariff grits, etc. | Sandstone and conglomerate. |
A glance at this table suffices to show that when we read Hugh Miller’s graphic descriptions of the Old Red Sandstone of Scotland, with its numerous and wonderful fishes, we have before us a formation altogether distinct from that of Devonshire or the Eifel. But the one represents the shallow, and the other the deeper seas of the same period. We learn this by careful tracing of the beds to their junction with, corresponding series, and by the occasional occurrence of the characteristic fishes of the Scottish strata in the English and German beds. In like manner a geologist who explores the Gaspé sandstones or the New Brunswick shales has under his consideration a group of beds very dissimilar from that which he would have to study on the shores of Lake Erie. But here again identity of relations to the Silurian below and the carboniferous above, shows the contemporaneousness of the beds, and this is confirmed by the occurrence in both series of some of the same plants and shells and fishes.
It will further be observed that it is in the middle that the greatest difference occurs. Sand and mud and pebble-banks were almost universal over our two great continental plateaus in the Older and Newer Devonian. But in the Middle there were in some places deeper waters with coral reefs, in others shallow flats and swamps rich in vegetation. Herein we see the greater variety and richness of the Devonian. Had we lived in that age, we should not have seen great continents like those that now exist, but we could have roamed over lovely islands with breezy hills and dense lowland jungles, and we could have sailed over blue coral seas, glowing below with all the fanciful forms and brilliant colours of polyp life, and filled with active and beautiful fishes. Especially did all these conditions culminate in the Middle Devonian, when what are now the continental areas of the northern hemisphere must have much resembled the present insular and oceanic regions of the South Pacific.