sage of their waters through the outlets; when this has been effected by distinct stages, successive terraces are formed; this is supposed to have been the origin of the lake ridges -ridge roads-they having constituted the ancient shores of the lakes. The circumstances of deposition of the freshwater beds of the Tertiary, Wealden, and Coal series appear to have been similar to those of lacustrine accumulations of the present era. Rivers have sometimes deposited their silt upon their beds, and more frequently during freshets upon the adjacent valleys; such deposits are more heterogeneous and irregular than those made in lakes.

308. Of the various mineral substances chemically precipitated from water, marl is the most abundant; it is the carbonate of lime held in solution and in mechanical suspension. It occurs in various degrees of purity; when densely aggregated and sometimes crystalline in texture,‘it is called rock marl; when cementing together a mass of shells, shell marl; and when largely mixed with clay, clay marl. Marls are found most frequently in the deposits of lakes, ponds and swamps in limestone districts, in which calcareous springs abound. Immense quantities of this material are also conveyed by rivers into the ocean. "A hard stratum of travertin,” says Mr. Lyell, "about a foot in thickness, is obtained from the waters of San Filippo in four months; and as the springs are powerful, and almost uniform in the quantity given out, we are at no loss to comprehend the magnitude of the mass which descends the hill. which is a mile and a quarter in length, and the third of a mile in breadth, in some places attaining a thickness of two hundred and fifty feet. To what length it might have reached it is impossible to conjecture, as it is cut off by a stream which carries the remainder of the calcareous matter

to the sea." Waters holding silica in solution-hot springs -have deposited silicious sinter; the hot springs of St. Michael have encrusted surrounding objects, and deposited layers of sinter several inches thick. Alumina is also sometimes similarly precipitated; a mixture of precipitated alumina, silica and the oxide of iron has been obtained abundantly from Tripoli for use as a polishing powder but much of the tripoli of commerce consists of the silicious shields of animalcules. The oxides of iron and manganese, gypsum, common salt, petroleum or asphaltum, etc., have formed extensive deposits; they furnish many analogies by which the phenomena of similar beds in the older strata are illustrated.

309. The plants embedded in the alluvium belong to existing species, and occur as subterranean forests, peatmosses, and drift-wood. Subterranean forests occur in depressed valleys, and low alluvial plains, where trees standing erect or overturned in the situations in which they grew have been invested with mud and sand; the wood of these trees, though discolored, has been employed in the construction of houses in England. Rivers, ocean-currents, and tides have formed extensive accumulations of driftwood in estuaries and deltas, and along the sea coast, par ticularly in sheltered bays. An instance of alluvial vegetable accumulation has been discovered by a section of a canal in Scotland. "At a depth of twelve feet from the surface of the fine alluvial sediment," says Professor Phillips, "a quantity of hazel bushes, roots, and nuts, with some mosses, fresh-water shells, and bones of the stag were met with. In some parts of the sediments an English coin was found, and oars of a boat were dug up. water entered this peaty and shelly deposit

Where a little from the adja

cent limestone, it produced in the wood a singular petrifaction; for the external bark and wood were converted into carbonate of lime, in which the vegetable structure was perfectly preserved. In like manner some of the nuts were altered; the shells and the membranes lining it were unchanged; but the kernel was converted into the carbonate of lime, not crystallized, but retaining the peculiar texture of the recent fruit. In this particular case no reasonable doubt can exist that the peaty deposit, full of land-mosses, hazel-bushes, and fresh-water-shells, was water-moved, and covered up by fine sediments from the river and the tide."

The

310. But the most extensive vegetable deposits of the alluvium are peat-mosses, which cover hundreds of square miles, and are sometimes forty feet thick. Peat consists of mosses, especially the sphagnum palustre, rushes and other aquatic plants, together with the trunks, branches, and leaves of trees. Peat swamps occupy the sites of ancient lakes and low woody districts, in which obstructions to the drainage have caused swampy morasses, destroying the forest trees and favoring the growth of aquatic mosses. sites of many of the aboriginal forests of Europe are now covered by mosses and fens. Fallen trees by obstructing the drainage of a district have produced peat-bogs; the prostration of a forest by a tornado about the middle of the seventeenth century produced a peat-moss, which at the beginning of the eighteenth century yielded peat for fuel. Peat swamps possess eminent antiseptic power; the bodies of men and other animals buried in them have been preserved for centuries. The most ancient peat-mosses belong to the alluvial period; this is known by their conforming to the present configuration of the land, and by their containing the remains of vegetable and animal bodies belonging

exclusively to existing species. The changes which occur in peat-beds illustrate the formation of coal; a true bituminous coal has been found in a peat-bog in the State of Maine, several feet below the surface amidst the remains of logs of wood, (§ 144.)

311. The remains of animals belonging to the species still living or very recently extinct, are characteristic of the Alluvium; among these remains the exuviæ of shellfish and coral zoophytes are, as they were in the older formations, most abundant. Immense banks of dead shells have been drifted together by tides and currents; some of Fig. 161.

Shell Limestone from the Mouth of the Thames.

them remain loose and are worn by the waves, while others are cemented together by the carbonate of lime into a shelly limestone or sandstone. Fig. 161 represents a portion of a block taken from a bank of consolidated shells, in the progress of formation in the English channel near the mouth of the Thames; it is sufficiently firm to admit of being cut and polished; the bank consists mostly of one species, having lived gregariously, as do oysters and mus

sels.

Coral reefs with their accumulated debris constitute thousands of square miles of the surface of the earth, (§ 76,) and form an aggregate of calcareous matter equal to the limestones of the older formations. The remains of fishes, reptiles, birds and mammalia, which are included in alluvial deposits, though comparatively few in number, are highly indicative of the circumstances in which they lived.

312. Some genera and species of animals have become extinct during the alluvial period; this may have been the case with the great Irish Elk, the Mastodon, the Dinornis and others; but there is one example of extinction which has occurred within the period of authentic history. The Dodo, a bird of the gallinaceous tribe larger than a turkey, abounded in Mauritius and adjacent islands, when first colonized by the Dutch, more than two centuries ago. It is now entirely extinct. The stuffed skins formerly in European Cabinets have decayed, and the only relics of them are a few fragments of the harder parts, as the head and feet, in the British Museums. The

bones of the Dodo have been found fossil in a tufaceous deposit in the Isle of

France. The Apteryx, so called because destitute of wings, is a rare bird living in New Zealand. It differs from most birds in many particulars of its organization. It appears to be almost extirpated, and several of its congeners are thought to have recently become extinct.