Lyell had studied also the Danish "kitchen-middens," familiar to those who have been carefully over the museums at Copenhagen. These shell-mounds, the refuse heaps of this ancient race, are sometimes one thousand feet long and two hundred wide. As far back as the time of the Romans the Danish isles were covered with magnificent beech forests. In the bronze age there were no beech trees, but oaks. In the stone age the Scotch fir prevailed, and thousands of years must have elapsed while these giant forests succeeded each other. The delta and alluvial plain of the Mississippi Lyell found to consist of sediment covering an area of thirty thousand square miles, several hundred feet deep. Taking the amount deposited annually, it would require from fifty to one hundred thousand years to produce the present deposits. The coral reefs of Florida, built up at the rate of one foot in a century, each reef adding ten miles to the coast, have required, according to Agassiz, at least one hundred and thirty-five thousand years for building. Human remains in a bluff on the shores of Lake Monroe, in Florida, he shows to be at least ten thousand years old. Under the streets of Glasgow, Scotland, seventeen canoes have been dug up, one in a vertical position, as if it had sunk in a storm, with the prow uppermost. Twelve canoes one hundred yards back from the river were found nineteen feet beneath the surface. Almost all were single oak trees, hollowed out by blunt tools, probably stone axes, aided by fire, relics of the stone age. In caverns near Liège, France, human bones have been found, with the cave-bear, elephant, rhinoceros, and other species now extinct. Skulls found in these primeval caves, especially one near Düsseldorf, called the "Neanderthal," "is the most brutal of all known human skulls, resembling those of the apes." These rude men probably were living at the same time, or even later, than the makers of the "refuse heaps" of Denmark. Wales has been under the sea to the depth of fourteen hundred feet, as proved by glacial shells; its submergence and reëlevation would require, by careful computation, about two hundred and twenty-four thousand years. Lyell showed that the Alps, Andes, and Himalaya Mountains were all elaborated under water. "The Alps have acquired four thousand, and even, in some places, more than ten thousand feet of their present altitude since the commencement of the Eocene (dawn of recent) period.... It is not too much to say that every spot which is now dry land has been sea at some former period, and every part of the space now covered by the deepest ocean has been land. The present distribution of land and water encourages us to believe that almost every conceivable transformation in the external form of the earth's crust may have been gone through. In one epoch the land may have been chiefly equatorial; in another, for the most part polar and circumpolar."