animal remains; for portions, says Mr. Hunt, of the tissues of various marine animals of low grade are destitute of nitrogen, and very similar in mineral composition to the woody fibre of plants. The probability, in some cases, of an animal origin has been especially inferred from the frequency of what is called anthracite in the calciferous beds' of the Lower Silurian of New York. The anthracite of this ancient rock, says Mr. Hunt, is an inspissated mineral oil. Petroleum is found in formations of every age, from the Lower Silurian up to the Tertiary; but a large number of the oil-wells of the United States, which have lately attracted so much attention, are in Devonian rocks.

In some instances the petroleum appears to filter slowly into the wells from the porous strata around, which are saturated with it, while, at other times, the boring instrument seems to strike upon a fissure communicating with a reservoir which furnishes at once great volumes of oil.*

The great pitch lake of Trinidad is situated, according to Mr. Wall, in Tertiary strata, chiefly Upper Miocene, but partly, perhaps, Lower Pliocene. The asphalt is derived from bituminous shales, containing vegetable remains, which are sometimes seen in the process of transformation, with their organic structure more or less obliterated. Occasionally the bituminous substance becomes plastic and even oily, and rises to the surface.† Such changes from oil to a pitch may be brought about, says Mr. T. S. Hunt, partly by the evaporation of the volatile ingredients, and partly by oxidation from the air.

Captain Mallet observes that, near Cape La Braye, in the island of Trinidad, fluid bitumen sometimes oozes out from the bottom of the sea, and rises to the surface. The same author quotes Gumilla, as stating, in his 'Description of the Orinoco,' that' about seventy years ago, a spot of land on the western coast of Trinidad, near half-way between the capital and an Indian village, sank suddenly, and was immediately replaced by a small lake of pitch, to the great terror of the inhabitants.'I

*Sterry Hunt, Canadian Naturalist, vol. vi. p. 245. August 1861.

Wall. Quart. Geol. Journ. vol.

xvi. p. 468. 1860.

Mallet, cited by Dr. Nugent, Geol. Trans. vol. i. p. 69. 1811.

A similar subsidence, at an earlier period, may probably have given rise to the great pitch lake of Trinidad, the cavity having become gradually filled with asphalt. Every geologist is familiar with the odour emitted from what are called fetid limestones when first broken. The Niagara limestone of the Upper Silurian group in America is sometimes so impregnated with bitumen, that this substance, when the stone is burned for lime, flows from the kiln like tar.*

*T. S. Hunt, ibid. p. 245.

416

CHAPTER XVIII.

REPRODUCTIVE EFFECTS OF RIVERS.

LAKE DELTAS-GROWTH OF THE DELTA OF THE UPPER RHONE IN THE LAKE OF GENEVA-PLAYFAIR ON THE ORIGIN OF LAKE-BASINS-COMPUTATION OF THE AGE OF DELTAS-RECENT DEPOSITS IN LAKE SUPERIOR-DELTAS OF INLAND SEAS-COURSE OF THE PO-ARTIFICIAL EMBANKMENTS OF THE PO AND ADIGE-DELTA OF THE PO, AND OTHER RIVERS ENTERING THE ADRIATIC-RAPID CONVERSION OF THE GULF INTO LAND-MINERAL CHARACTERS OF THE NEW DEPOSITS-MARINE DELTA OF THE RHONE-VARIOUS PROOFS OF ITS INCREASE-STONY NATURE OF ITS DEPOSITS -COAST OF ASIA MINOR -DELTA OF THE NILE-CHRONOLOGICAL COMPUTATION OF THE GROWTH

OF THE NILE MUD AT MEMPHIS.

DELTAS IN LAKES.

I HAVE already spoken in the fourteenth chapter of the action of running water, and of the denuding power of rivers, but we can only form a just conception of the excavating and removing force exerted by such bodies of water, when we have the advantage of examining the reproductive effects of the same agents: in other words, of beholding in a palpable form the aggregate amount of matter, which they have thrown down at certain points in their alluvial plains, or in the basins of lakes and seas. Yet it will appear when we consider the action of currents, that the growth of deltas affords a very inadequate standard by which to measure the entire carrying power of running water, since a considerable portion of fluviatile sediment is swept far out to sea.

Deltas may be divided into, first, those which are formed in lakes; secondly, those in inland seas, where the tides are almost imperceptible; and, thirdly, those on the borders of the ocean. The most characteristic distinction between the lacustrine and marine deltas consists in the nature of the organic remains which become imbedded in their deposits;

for, in the case of a lake, it is obvious that these must consist exclusively of such genera of animals as inhabit the land or the waters of a river or lake; whereas, in the other case, there will be an admixture, and most frequently a predominance, of animals which inhabit salt water. In regard, however, to the distribution of inorganic matter, the deposits of lakes and seas are formed under very analogous circumstances.

Lake of Geneva.-Lakes exemplify the first reproductive operations in which rivers are engaged when they convey the detritus of rocks and the ingredients of mineral springs from mountainous regions. The accession of new land at the mouth of the Rhone, at the upper end of the Lake of Geneva, or the Leman Lake, presents us with an example of a considerable thickness of strata which have accumulated since the historical era. This sheet of water is about thirtyseven miles long, and its breadth is from two to eight miles. The shape of the bottom is very irregular, the depth having been found by late measurements to vary from 20 to 160 fathoms.* The Rhone, where it enters at the upper end, is turbid and discoloured; but its waters, where it issues at the town of Geneva, are beautifully clear and transparent. An ancient town, called Port Vallais (Portus Valesiæ of the Romans), once situated at the water's edge, at the upper end, is now more than a mile and a half inland—this intervening alluvial tract having been acquired in about eight centuries. The remainder of the delta consists of a flat alluvial plain, about five or six miles in length, composed of sand and mud, a little raised above the level of the river, and full of marshes.

Sir Henry De la Beche found, after numerous soundings in all parts of the lake, that there was a pretty uniform depth of from 120 to 160 fathoms throughout the central region, and on approaching the delta, the shallowing of the bottom began to be very sensible at a distance of about a mile and three quarters from the mouth of the Rhone; for a line drawn from St. Gingoulph to Vevey gives a mean depth of * De la Beche, Ed. Phil. Journ. vol. ii. p. 107. Jan. 1820. VOL. I.

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somewhat less than 600 feet, and from that part to the Rhone, the fluviatile mud is always found along the bottom.* We may state, therefore, that the new strata annually produced are thrown down upon a slope about two miles in length; so that, notwithstanding the great depth of the lake, the new deposits are inclined at so slight an angle, that they would be termed, in ordinary geological language, horizontal.

The strata probably consist of alternations of finer and coarser particles; for, during the hotter months from April to August, when the snows melt, the volume and velocity of the river are greatest, and large quantities of sand, mud, vegetable matter, and drift-wood are introduced; but, during the rest of the year, the influx is comparatively feeble, so much so, that the whole lake, according to Saussure, stands six feet lower. If, then, we could obtain a section of the accumulation formed in the last eight centuries, we should see a great series of strata, probably from 600 to 900 feet thick, and nearly two miles in length, inclined at a very slight angle. A much more considerable deposit of similarly stratified matter, of an age antecedent to the historical, would be seen extending to the original head of the lake five or six miles distant from the accumulations of the last 800 years. Simultaneously with the growth of the principal delta, a great number of rapid torrents are bringing down large masses of sand and pebbles, and forming smaller deltas at their mouths round the borders of the lake. The body of water in such torrents is too small to enable them to spread out the transported matter over so extensive an area as the Rhone does. Thus, for example, there is a depth of eighty fathoms within half a mile of the shore, immediately opposite the great torrent which enters east of Ripaille, so that the dip of the strata in that minor delta must be about four times as great as that of the deposits formed by the main river at the upper extremity of the lake.†

The capacity of this basin being now ascertained, it would be an interesting subject of enquiry, to determine in what