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changes that must have taken place, and the ages that must have elapsed since the gravels and sands were piecemeal worn and washed down from their parent formations.*
Closely associated with these river-deposits, and sometimes indeed undistinguishable from them, are the Lacustrine or lake deposits, that occupy so many of the alluvial expanses in our plains and valleys. The tendency of every lake fed by running streams is to become shallower and shallower from the sediment deposited in its basin by these inflowing waters. Every stream protrudes its little delta of silt and sand, fresh-water shell-fish accumulate layers of marl, and aquatic plants contribute their annual quota of growth and decay. By-and-by the shallow lake becomes a stagnant morass, and in process of time, partly by surface plant-growth, and partly by the deepening of the outflowing stream, the morass is converted into meadowland. A large portion of all the "straths" of Scotland and the "dales" of England are of lacustrine formation; and we have only to watch the cutting of any main drain
* The following is a section of the Uralla gold-field, as given to us some years ago by Mr W. Cleghom of that district:— Red rich soil, Stiff red clay,
Mottled clay—volcanic ashes,
numerous crystal pebbles and a little gold,
Loose sand (decomposed quartz and granite), with
numerous pebbles—the main gold deposit, . 4
Granite, water-worn surface, with large granitic boulders.
through their subsoils to be convinced of the truth of this origin. A thick layer of vegetable or peaty soil, followed by beds of silty sand, marl, and clay, embedding the bones of deer, oxen, and other animals, with the remains of an occasional tree-canoe, clearly bespeak their lacustrine formation, and point to the time when the wild animals of the country were mired in their muds, and the primitive inhabitants paddled across their waters. Now how changed ! the site of the former lake green with the richest pastures, or waving with luxuriant corn-fields! As with the straths and dales of Britain, so with a large proportion of all the inland plains and valleys of the world. Many of them are but chains of silted-up lakes converted into dry land, partly by the process of silting or filling up, and partly by the main stream of the valley cutting deeper and deeper its channel, and thus affording a more thorough drainage for the whole. Lacustrine formations, though occurring in the same plain with those of fluviatile origin, are in general readily distinguishable by their finer sediments, greater regularity of deposition, the occurrence of beds of shell-marl and peat, and the more perfect preservation of their organic remains. These remains are often of great antiquity, ranging from the time of the mammoth, great Irish deer, and species of oxen that have been long extinct, down to the pile-dwellings of our Celtic or pre-Celtic ancestors, who betook themselves for safety to their waters, and erected artificial mounds for their habitations, where nature had not provided the necessary "inches" or islands.* Even since the time of the
* Since 1854 these lake - dwellings or pile-dwellings (known as pfaklbauten in Switzerland, and crannogea in Ireland) have received much attention from archaeologists and geologists. These dwellings occur in existing lakes, as well as in bogs and marshes which were formerly the sites of lakes, and seem to have been erected on piles driven through the water, or on mounds partly formed of stones, wood, and other debris. They have been found in Switzerland, Ireland, Scotland, and
earlier Celts, hundreds of shallow lakes and morasses have been converted into dry land; and the process still goes forward, accelerated in all civilised countries by the incessant operations of man.
Still associated with river-action, but necessarily separated from strictly fluviatile deposits, are those Estuarine formations which occupy extensive areas in almost every region of the globe. Wherever a river discharges itself into the sea by a broad mouth, or by many mouths, and in particular where the tidal influence pounds back the river-water and runs for some distance inland, sand-banks and mud-shoals have a tendency to accumulate. In process of time the banks and shoals become islands, and by further accretion and union the islands are converted into deltas. In this way most of our larger rivers present deltic flats or estuarine formations, and these must have been slowly accumulating since sea and land received their present relative configurations. These accumulations, consisting mainly of debris borne down by the river, but partly also of tidal sediments, will embed marine as well as terrestrial and
other European countries, and point to a time when the early inhabitants betook themselves to this style of habitation for purposes of defence and protection. In some instances, as in the Swiss lakes, the piled area is of considerable extent (forming an aquatic village, as it were), and connected with the shore by a piled way or causeway. In the older pfahlbauten the implements are chiefly of stone, and associated with the castaway bones of the deer, boar, and wild-ox; in those of intermediate age, bronze implements prevail, associated with the bones of the domestic ox, pig, and goat; while in the more recent, iron swords and spears hare been found, accompanied by carbonised grains of wheat and barley, and with fragments of rude textures woven of flax, bast, and straw. The more recent seem to have been immediately anterior to the great Roman invasion of northern Europe; the more ancient may be several thousand years older than that event. Those curious in these matters may consult Dr Keller's 'Lake-Dwellings of Switzerland,' as translated and arranged by Mr Lee, 1866.
fresh-water organisms, and thus they are regarded as estuarine or fluvio - marine, in contradistinction to those of strictly fluviatile or lacustrine origin. The low-lying deltas of the Mississippi, the plain of Lower Egypt, the jungleswamps of the Niger, the sunderbunds or mud-islands of the Ganges and Irawaddy, and the alluvial plain of China, are familiar examples on a great scale of these estuarine or deltic deposits; but as with these, so with almost every other river that discharges its waters into the ocean. The magnitude of estuarine formations is one of the most notable features in the geology of the current epoch, and this magnitude is increased by a twofold process which the reader would do well to consider.
In the first place, the delta that makes its appearance as dry land may form but a small portion of the sedimentary matter borne down by a river, the greater portion being carried forward and projected, as it were, over the bed of the ocean. An estuarine formation is thus partly subaerial and partly submarine, and it necessarily requires a long and gradual process of silting to convert the submarine into sub-aerial. But during the oscillations or crust-motions to which the earth is subjected, it frequently happens that a whole island or portion of a continent is gradually upraised, and thus the submarine portion of an estuary may be upheaved into dry land, and this altogether independent of the slow and ordinary process of silting. An uprise of fifty feet would convert a large portion of the Yellow Sea into a lower terrace of the great Chinese plain; and by a similar uprise thousands of square miles of Mississippi swamp would assume the character of fertile prairie-ground. To such upheavals much of the "carses" of Scotand and "levels" of England are no doubt due; for though wholly composed of river and marine silts, their final conversion into dry alluvial plains has been more a matter of terrestrial uprise than of sedimentary accumulation. As with the estuarine plains of our own islands, so to a great extent with those of other regions: they owe their accumulation wholly to silt and sediment, but their conversion into dry land, partly to silting, and partly to terrestrial upheavals.
Whatever their mode of accretion, the composition of these estuarine formations is much the same in every region: mud-silts, clays, sands, gravels, drift-wood, shellbeds, peat and swamp earths—the whole being usually surmounted by loamy vegetable soils of extraordinary fertility. Their embedded remains are partly terrestrial, partly fresh-water, and partly marine; and these, of course, will differ according to the latitudes in which the estuary occurs, and the regions through which its affluent rivers flow. Thus the Mississippi will sweep down the terrestrial and fresh-water spoils of temperate North America, the Amazon those of tropical South America, the Niger those of equatorial Africa, and the Ganges and Irawaddy those of subtropical Asia. Every estuary, in fact, is characterised by its own fossil flora and fauna, and these of varying antiquity, from the spoils swept down by the latest land-flood or deposited by yesterday's tide, back to the confines of the glacial epoch, if in the higher latitudes, and it may be to the tertiary itself, if occurring in intertropical regions. In the estuarine silts of our own islands, for example, we pass through every gradation of antiquity, from the plants and animals now flourishing around us, back through those which, like the bear, wild-boar, wolf, and beaver, have long since been extirpated, and from these backwards still to the seals, whales, and boreal shells that inhabited our firths and estuaries in times immediately post-glacial. The reader may readily trace this gradation in the estuarine deposits of the Clyde, Forth, Tay, or any other of our