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in lithological and in faunal characters—a lower bed, which is essentially a littoral clay, and is known as the Scrobicularia zone, and an upper bed, which has been deposited in at least five fathoms of water, and which Stewart has named the Thracia eonvexa zone, from the abundance of that bivalve at Belfast. At Belfast, Magheramorne, and Downpatrick, both zones are represented. The deposits at Bann, Kilroot, and Newcastle are typical Scrobicularia clays; the Larne, Newtownards, and Kircubbin beds are intermediate ; while the Lough Foyle and Greenore clays show less characteristically the Thracia zone.
The method employed in determining approximately the depth of water in which any bed was laid down, was to select a number of its most abundant and characteristic shells, and note the minimum depths in which those species now live in local waters, tbe upper bathymetrical limit being usually more constant and better known than the lower; by taking the average of these minima, a minimum depth is arrived at which is in all likelihood pretty near the mark. Tried by this criterion, it would appear that the Thracia clays of Magheramorne, Belfast, and Downpatrick, accumulated in some five fathoms minimum; the Lough Foyle beds and Greenore in two to three fathoms; Larne and Kircubbin in one to two fathoms; the other in the littoral zone.
The Scrobicularia clay typically consists of brownish-blue, somewhat sandy clay, containing abundance of the roots and leaves of the grass-wrack, Zostera marina, and charged with a vast number of shells belonging to a comparatively limited number of species which have their habitat between tide-marks. The following are characteristic:— Mytilus edulis, Cardium edule, Tapes decussatus, Tellina balthica, Scrobicularia piperata, Hydrobia ulvas. The upper or Thracia clay is remarkably pure, fine, and unctuous, and light blue in colour. Its fauna is less in actual numbers, but much greater in variety, and is characterized by shells belonging to the laminarian and coralline zones, such as Montacuta bidentata, Cardium echinatum, Lucinoptii undata, Scrobicularia alba, Thracia eonvexa, Turritella terebra. The raised beaches which fringe our north-eastern shores are in general contemporaneous with these deep-water clays, and frequently, like 1 them, repose on clays of the Scrobicularia zone; they (the raised beaches) do not come within the scope of the present Paper, but it may be remarked that at Larne the well-known Curran gravels overlie the estuarine clay to a depth of twenty feet, containing flint implements and marine shells to their base; at Kilroot, the raised beach which
has been described by Professor Hull,1 rests on Scrobicularia clay; at Ballyholme, the blackish zone overlying the submerged peat, which represents the estuarine clay, is topped by twenty-two feet of marine gravels; and at Greenore and Dundalk stratified shelly gravels fifteen feet in thickness cover the clay. Occasionally the uppermost bed is of a different nature. Thus, at the Bann the Scrobicularia clay is covered with a considerable thickness of stratified brownish river sand representing the Thracia zone; and at another spot by six feet of blown sea-sand; and at Belfast (Alexandra Dock), resting on the deep-water clay, there is a bed of yellow sand full of shells washed out of the underlying stratum, capped by some six feet of littoral clays, still in course of formation, and crowded with Mya armaria and Cardium edule.
The fauna which characterizes the estuarine clays shows that a long period must have elapsed between the close of the Glacial epoch and the deposition of even the lowest bed of the series, and what a length of time the intervening beds of red sand, clay, and gravel represent. Whereas the Boulder clay is characterized by shells which flourish amid the rigours of an Arctic climate, the estuarine clay fauna presents, if anything, a rather more southern aspect than that now existing in local waters. In other words, while the species found in the local Boulder clays which do not now exist on our present shores are generally of northern types, the corresponding forms yielded by the estuarine clays have usually now their habitat further southward. But there is very little in the latter point; for the estuarine clay fauna differs to no material extent from that now existing within a short distance.
The changes of level shown by the series above described are instructive. Far back in Post-glacial times, but long after the grand era of depression and elevation which characterized the Great Ice Age, the land must have stood at least thirty feet higher than at present. Low swamps fringed the shores, with jungles of reeds and sedges, and copses of willows, alders, and hazels, among which roamed the red deer, the wild boar, and the great Irish elk.
A period of gradual depression ensued, and submergence of the land followed, with a slow accumulation of littoral mud, in which lived thousands of burrowing inter-tidal bivalves. A more rapid further depression followed, and in a depth of five or ten fathoms the
1 Eeport of British Association, 1872.
deposition of fine impalpable sediment continued, and at spots -where tidal currents ran with force, or at the mouth of streams, sandbanks and gravel accumulations were formed. It is not easy to account for the presence of estuarine clay in certain spots. In the estuaries of rivers of some volume, such as the Loughs of Foyle and Belfast, and the estuary of the Quoilc, and Dundalk Harbour, where the quantity of transported sediment is an appreciable factor, one is not surprised to find beds of silt; but river-borne mud will not account for such deposits as the estuarine clays of Magheramorne, Kilroot, and Newcastle, where, in the first-mentioned locality, thick beds have been formed where no streams of appreciable volume occur, and in the lastmentioned station a deposit of typical estuarine clay occurs in a wide exposed sandy bay. Probably Mr. Mac Adam is right when he suggests 1 that growths of wrack—in these cases ZotUra marina—entangling and retaining the muddy particles, contributed to the formation of the clays. In order to test whether the deposits at present forming between tides in our estuaries resemble the Scrobicularia clays, a sample was carefully taken from two feet below the surface near Ballycarry on Larne Lough, where extensive mud-flats occur. On examination, it proved a bluish, very sandy mud, entirely without shells, and only containing a few starved specimens of three or four of the commonest Foraminifera, differing materially, therefore, from deposits of the estuarine clay series. The surface clays at the Alexandra Dock had more in common with the Scrobicularia zone; but were much less homogeneous in composition. A deposit more resembling the estuarine clays was discovered by Professor Dickie in his zoological exploration of Strangford Lough.5 The entrance to this extensive sheet of water is much contracted, and through it the tides run with immense force. A series of dredgings taken in the Irish Sea opposite the entrance to the Lough showed that while coarse gravel formed the bottom at and near the bar, the material became finer as the distance from shore increased, and seven miles from the bar, in a depth of twenty-six fathoms, the deposit consisted of fine black tenaceous mud, in which Scrobicularia nitida lived. The colour of the estuarine clays is uniformly bluish, whereas the recent deposits which have been compared to them are black; but it may be remarked that a very black, unsavoury surface clay, full of Hydrobia ultce, encountered at the
1 Op. cit. p. 262.
5 Report of British Association, 1857.
Alexandra Dock, turned light blue on the surface on twenty-four hours' exposure to the air; so that probably the blue colour of the estuarine clays is due to gradual oxidation.
Following this period of submergence, an era of elevation set in, ■which raised the surface to its present level. The deep-water clays emerged from the sea, or remained a few feet below its surface, and the sands and gravels were elevated, in places, to twenty or thirty feet above high-water mark. In some cases the Thracia clays may have been removed by marine denudation as the land rose, and the raised beaches1 probably suffered considerably from the same cause. Since the upheaval, littoral clays and sands have been locally thrown down, forming the newest zone of the series. I shall not attempt to correlate the fluctuations of level which have been above described with those which have been or may be' deduced from observations of the contemporaneous beds of the rest of Ireland, and of England and Scotland. I shall perform a more useful service, by accurately detailing that which has come directly under my own notice, than by entering into generalizations, which continuous and careful observation over an extended area alone would justify or make valuable; but I may remark that a succession of oscillations of level closely corresponding to those above described has been deduced by Rev. Maxwell H. Close from the observed levels of raised beaches and submerged peat in the Dublin district.*
The opportunities afforded for the study of the estuarine clays are few and far between, since the deposits lie almost without exception below tide-level, and are consequently inaccessible except when excavations or other artificial means permit their examination; such chances as are afforded by harbour works, and foundations of bridges and buildings, require to be promptly taken. Even then, the difficulties of the investigator are not at an end, for the clays are wet, soft, and tenacious, and the state of one's garments, hands, and feet, after a few hours of fossil-hunting, is not inviting. It is impossible to pick the smaller forms out of the clay in its natural condition, and a more or less troublesome process must be undergone before the small shells and microscopic forms can be examined. The method adopted in the present case is as follows :—The clay is slowly dried, by which process
1 The term "raised beach" is used in a wide sense in the present Report; in many cases the term "beach" is not strictly applicable, the deposits being sea-beds, «s proved by the molluscs in situ which they contain.
1 Guide to the County of Dublin, 1878, p. 45.
it loses about one-third of its weight. "When quite dry and hard it is thrown into cold water, and allowed to soak for from one to twentyfour hours; the more tenacious clays requiring a longer time than sandy clays. This produces pulverization of the clay, and entirely does away with its plastic character; but where vegetable matter is present, several successive dryings and soakings may be necessary. It is then washed through a sieve of 150 meshes to the square inch, which arrests the coarse matter and larger shells. A sieve of 900 meshes per square inch is next employed; it arrests the smaller shells, allowing microscopic forms to pass through. A sieve of very fine silk of 36,000 meshes per square inch is then used, which retains all Foraminifcra, Ostracoda, &c, permitting the passage of only the impalpable sediment which composes the clay. When dried, the first siftings can be examined with the naked eye, the second by aid of a good lens, while for the Foraminifera, &c, the microscope is, of course, employed. For the purposes of the present report, over 300 lbs. of cstuarine clay have been treated in this manner. The proportion of organic matter which the clays yield varies much. In some forty selected samples examined by the writer, the highest percentage of shells in the clay (dry) was "1 at Eglinton, and the lowest •001 at Newcastle, or from one-tenth to one-thousandth; but at Magheramorne some narrow zones of the clay consist of almost pure shells, and in others shells are entirely absent.
The exuberant growth and abundance of shells in the estuarine clays has been noticed by all the writers on the subject, and these points are made conspicuous by the beautiful state of preservation of the fossils in the majority of the deposits, the close and impervious nature of the clay protecting its embedded remains from the destroying effects of both air and water. The huge size which Pkolas critpata attains has been a subject of frequent comment. Specimens obtained in the Pholad zone at Alexandra Dock wero frequently 4J- inches broad, with a girth of 8 inches.
Ostrea attains 6 inches long by 5 broad; Pecten varitu, 2f inches by 2i, at both Belfast and Magheramorne; a P. maximtu from Magheramorne measured 6 inches by 7; specimens of Tapes virgineus, with a breadth of 2 to 3 inches, are abundant in the Magheramorne deposit; a single valvo of Gastrana found at Downpatrick was 1J inches by 2J; Scrobicularia piperata reaches in several deposits 1J inches by 1\; specimens of Thracia distorta found at Magheramorne measured | inches by 1 \; and Turritella of from 2 J inches to 2J long occur in several beds.