The high tides of the Bay of Fundy, rising more than 60 feet, are so destructive as to undermine and sweep away continually the whole face of the cliffs, and thus a new crop of erect trees is brought into view every three or four years. They are known to extend over a space between two and three miles from north to south, and more than twice that distance from east to west, being seen in the banks of streams intersecting the coal-field.

In Cape Breton, Mr. Richard Brown has observed in the Sydney coal-field a total thickness of coal-measures, without including the underlying millstone grit, of 1843 feet, dipping at an angle of 8°. He has published minute details of the whole series, showing at how many different levels erect trees occur, consisting of Sigillaria, Lepidodendron, Calamite, and other genera. In one place eight erect trunks, with roots and rootlets attached to them, were seen at the same level, within a horizontal space 80 feet in length. Beds of coal of various thickness are interstratified. Some of the associated strata are ripple-marked, with impressions of rain-drops. Taking into account forty-one clays filled with roots of Stigmaria in their natural position, and eighteen layers of upright trees at other levels, there is, on the whole, clear evidence of at least fifty-nine fossil forests, ranged one above the other, in this coal-field, in the above-mentioned thickness of strata.[324-A]

The fossil shells in Cape Breton and in the Nova Scotia section ([fig. 372.]), consisting of Cypris, Unio (?), Modiola, Microconchus carbonarius (see [fig. 375.]), and Spirorbis, seem to indicate brackish water; but we ought never to be surprised if, in pursuing the same stratum, we come to a fresh or purely marine deposit; for this will depend upon our taking a direction higher up or lower down the ancient river or delta deposit. When the Purbeck beds of the Wealden were described in Chap. XVIII., I endeavoured to explain the intimate connection of strata formed at a river's mouth, or in the tranquil lagoons of the delta, or in the sea, after a slight submergence of the land, with its dirt-beds.

In the English coal-fields the same association of fresh, or rather brackish water with marine strata, in close connection with beds of coal of terrestrial origin, has been frequently recognized. Thus, for example, a deposit near Shrewsbury, probably formed in brackish water, has been described by Sir R. Murchison as the youngest member of the carboniferous series of that district, at the point where the coal-measures are in contact with the Permian or "Lower New Red." It consists of shales and sandstones about 150 feet thick, with coal and traces of plants; including a bed of limestone, varying from 2 to 9 feet in thickness, which is cellular, and resembles some lacustrine limestones of France and Germany. It has been traced for 30 miles in a straight line, and can be recognized at still more distant points. The characteristic fossils are a small bivalve, having the form of a Cyclas, a small Cypris ([fig. 376.]), and the microscopic shell of an annelid of an extinct genus called Microconchus ([fig. 375.]), allied to Serpula or Spirorbis.

In the lower coal-measures of Coalbrook Dale, the strata, according to Mr. Prestwich, often change completely within very short distances, beds of sandstone passing horizontally into clay, and clay into sandstone. The coal-seams often wedge out or disappear; and sections, at places nearly contiguous, present marked lithological distinctions. In this single field, in which the strata are from 700 to 800 feet thick, between forty and fifty species of terrestrial plants have been discovered, besides several fishes and trilobites of forms distinct from those occurring in the Silurian strata. Also upwards of forty species of mollusca, among which are two or three referred to the freshwater genus Unio, and others of marine forms, such as Nautilus, Orthoceras, Spirifer, and Productus. Mr. Prestwich suggests that the intermixture of beds containing freshwater shells with others full of marine remains, and the alternation of coarse sandstone and conglomerate with beds of fine clay or shale containing the remains of plants, may be explained by supposing the deposit of Coalbrook Dale to have originated in a bay of the sea or estuary into which flowed a considerable river subject to occasional freshes.[325-A]

Freshwater Fossils—Coal.

Fig. 375.