338 ALLUVIUM vived by touching the Bones of Elijah," and " Uriel in the Sun," displaying high imagina- tive power and a rare mastery of color and chiaroscuro, obtained for the artist valuable prizes from the British institution, and all of them found ready purchasers. He returned to America in feeble health in 1818, and during the remainder of his life resided principally in Boston and Cambridge. His subsequent career was without the incentive to exertion which he had experienced in England. His country- men respected him for the reputation he had acquired abroad, but were scarcely able to ap- preciate his talents. Kemoved from the con- genial atmosphere of the great art capitals of the old world, he worked listlessly and ir- regularly, and produced no finished perform- ance of importance comparable in merit with his earlier pictures. During the last 25 years of his life he occupied himself from time to time on a composition of great size represent- ing " Belshazzar's Feast," which he intended should be his masterpiece. But frequent at- tacks of illness, an over-fastidiousness of taste, and an ideal which became more exalted and exacting as he advanced in years, seriously marred the progress of the work, and it re- mained at the close of his life an unfinished but splendid specimen of his genius. It is now the property of the Boston Athenaeum. All- ston's works are not numerous, considering the extent of his career, but bear the imprint of an original and artistic mind. The best are founded on Scriptural subjects. He also painted landscapes and sea pieces of great excellence, and in ideal portraits combined an almost un- rivalled purity of flesh tints with depth and power of expression. Had he possessed the moral courage and the physical ability to em- body on the canvas his own conceptions, he would have proved one of the most prolific and imaginative artists of the age. No Amer- ican painter has yet approached him in the de- lineation of sacred history. Allston was a man of fine literary tastes, and conversed with ease and eloquence on art and metaphysics. He published a volume of poems and a novel, " Monaldi," illustrating Italian life. ALLUVIUM (Lat., from alluere, to wash upon or against), the deposits of sand, gravel, marl, &c., brought down by running streams of the present geological period. Other recent ac- cumulations also, as those of peat and of the hills of sand blown together by the wind, are often called alluvial. They all belong to Lyell's uppermost group, the post-tertiary, and are characterized by containing human relics and remains. The same group comprises the cal- careous rocks of recent origin which occur on the coast of Guadeloupe, and contain human skeletons imbedded in solid limestone, and also the coral reefs which are in process of formation in tropical seas, spreading out in cal- careous strata hundreds of miles in extent. These are not usually included in the term al- luvium ; and yet it is not easy to draw a line that shall exclude any formations of recent origin ; for the wash of rivers, as it settles in the bays at their mouths, often finds some ce- menting matter that soon binds it into solid rock, and in this hard rock are entombed as fos- sils works of art or remains of man. Thus the term alluvium has no precise signification. The great deposits of alluvium accumulate so slowly and silently, that we little appreciate the immense changes made by running water upon the surface of the earth ; yet in the short period from the time to which our records ex- tend back, we find that the sediments of a few small Italian rivers have carried out the coast line into the gulf of Venice from 2 to 20 miles ; and that the ancient port of Adria, which in the time of Augustus gave its name to the gulf, is now an inland town, 15 miles from the shore. According to Herodotus, the ancient priests of Egypt regarded their country as " the gift of the Nile." From the great pyramids down to the sea all is made land. The great rivers of the world, as the Mississippi, Amazon, Ganges, and Orinoco, are producing effects far greater than those of the Nile ; but our obser- vations of these extend but a few generations back, and we lack sufficient data for calculating very exactly the rate of increase of their deltas. With the Mississippi, however, this has been attempted by Mr. Forshey, an eminent engineer, from observations extending through 80 years. Adopting the estimate of Dr. Riddell of New Orleans, that the weight of sediment is 18 * 48 of that of the water, or ^^ of its volume, and allowing the quantity of water brought down per second to be 447,199 cubic feet, the whole amount of sedimentary matter annually added to the delta and carried into the gulf is equal to 4,083,333,338 cubic feet, enough to cover 144 square miles one foot deep. And yet at this rate, for the river to have built up the great accu- mulations of alluvium which make its delta, would have required 61,000 years; and higher up there are the accumulations at this rate of some 30,000 years more. Thus long at least, it is probable, the great rivers have flowed as they now flow ; and during this latest epoch few changes have occurred in the lower forms of animal life; for in the strata next older than these alluvial deposits, the land and river shells are all of the same species with those now living in the same region. Subsequent investigations by Capt. Humphreys and Lieut. Abbot (1868) have given results which will be described under MISSISSIPPI RIVEB. The delta of the Ganges and Bramapootra is far more extensive than that of the Mississippi. It is a wilderness filled with a labyrinth of rivers and creeks, infested with tigers and crocodiles, and larger than the principality of Wales. The riv- ers pour down their turbid waters charged with sediment, and abounding with the ruins of ani- mal and vegetable life. These are swept into the bay of Bengal, the waters of which are discolored by the fine mud nearly 100 miles from its mouth, while the heavier materials
