That such changes have taken place in the past history of the river we have several interesting proofs. Thus, owing to the great earthquakes of 1811, 1812, an area of more than 2000 square miles was permanently submerged.[70] Since then it has gone under the name of the "Sunk Country;" and Sir Charles Lyell, who visited the locality in 1846, that is, thirty-four years afterwards, tells us that he saw innumerable submerged trees, some erect, others prostrate. Now, it is easy to see how such an area may, when the climate suits, become the receptacle of vast accumulations of peat, which, by pressure and chemical action, will ultimately pass into coal. If we suppose the submergence carried on more rapidly at some periods, the plants might have been unable to keep pace with the ever-increasing inroads of sand and mud. In such cases the layer of vegetation would become eventually entombed beneath succeeding deposits of earthy matter. Were the amount of sediment thus thrown down sufficient in the end to counteract the downward motion of the earth's crust, and so raise the bottom of the river or lake to the level of the water, vegetation would spring up afresh and clothe the new raised surface as densely as in former years. This alternation, according as the amount of sinking or the amount of sediment predominated, might go on for thousands of years, until a series of strata many thousand feet thick were accumulated, and tranquilly carried down bed after bed below the level of the waters. [Footnote 70: See Sir Charles Lyell's _Second Visit to United Stales_, chap, xxxiii.]