An equally conclusive proof of the immense antiquity of the glacial period is afforded by the formation known as the loess, which fills up so many of the valley systems of Europe, Asia, and America to great depths, and spreads over the adjacent table-lands. It is a tranquil land deposit of fine glacial mud, from sheets of water which inundated the country when great rivers from glaciated districts ran at higher levels, and began to excavate their present valleys. Lyell estimates the thickness of this deposit in the Rhine valley at 800 feet, and it is found at much higher levels on upland plains. Now this loess is not a marine or lacustrine deposit, as is proved by the shells it contains, which are all of land species; nor is it a deposit of running water, for there are no sands or gravels, but distinctly such a deposit from tranquil sheets of muddy water as is now accumulated in Egypt by the inundations of the Nile. When the Rhine brought down such volumes of muddy water from the glaciers of the Alps as to overflow the upland plains, it must have flowed at a level many hundred feet higher than its present valley, which must have been since scooped out by sub-aërial denudation. The rate of deposition of the Nile mud is about three inches per century, and there seems no reason why that of the fine glacial mud should have been more rapid, charged as the Nile is every year with mud from the torrential rains of the Abyssinian highlands. At this rate it would have required 320,000 years to accumulate the 800 feet of loess of the Rhine valley. Here again the rate may have been faster, but it is sufficient to show that an immense time must have elapsed, and the loess is a distinctly glacial deposit, containing palæolithic human remains and a pleistocene fauna, and embracing only a portion of the quaternary period. Nor is it an isolated phenomenon confined to Europe, but is found over the whole world wherever rivers have flowed from regions which were formerly buried under ice and snow. It is found in great force in the valleys of the Yang-tse-Kang and the Mississippi, and Sir Charles Lyell, referring to the fossil human bone found in it at Natchez, says--"My reluctance in 1846 to regard the fossil human bone as of post-pliocene date arose, in part, from the reflection that the ancient loess of Natchez is anterior in time to the whole modern delta of the Mississippi. The table-land was, I believe, once a part of the original alluvial plain or delta of the great river before it was upraised. It has now risen more than 200 feet above its pristine level. After the upheaval, or during it, the Mississippi cut through the whole fluviatile formation, of which its bluffs are now formed, just as the Rhine has in many parts of its valley excavated a passage through its ancient loess. If I was right in calculating that the present delta of the Mississippi has acquired, as a minimum of time, more than 100,000 years for its