(1.) The estimate of the time required for the alluvial deposits along the banks of rivers, has been extremely various. Lyell, having visited the delta of the Mississippi river in person, estimated its time-period of accumulation at one hundred thousand years.[14] But a careful examination made by gentlemen of the Coast Survey and other United States officers, reduces this time-period to four thousand and four hundred years.[15] Again, Mr. Lyell estimates that 220,000 years are necessary to account for changes now going on upon the coast of Sweden. Later geologists reduce the time to one-tenth of that estimate. A piece of pottery was discovered deeply buried under the deposits at the mouth of the Nile. It was confidently asserted that the deposits could not have been made during the historic period, until it was proved that the article in question was of Roman manufacture.[16] Such diversities suffice to show at least that somebody has blundered. Some of these high estimates are gratuitously extravagant. All estimates from the drift deposits, bearing on the antiquity of man, ought in reason to be made with careful reference to these two modifying considerations: (a.) That drift deposits may have been, and with the utmost probability were, much more rapid in the earlier ages than at present. At the close of the glacial and ice period vast masses of loose matter were ready to be swept rapidly as drift by river freshets. Any farmer may have an illustration of this if he will plow his side-hill field, running his furrows up and down the hill. He will find that the first powerful shower will bring down far more drift than the fortieth. It would be very short-sighted in him to take the drift of the tenth year after the said plowing for his rate of annual deposition and estimate the whole period from this data. But on this mistaken principle some geologists have made their time estimates for the drift simply monstrous. (b.) Human remains and tools may in many ways get far below the surface of the drift. They may have been buried under it after its deposition. While the drift lay under water, (soft and pliable therefore,) flints, arrow-heads, knives, or human bones, may have sunk in the mire.――――These and similar considerations may demand large abatement from the time-estimates built upon the amount of drift found above the remains of man.