The American geologists, after carefully studying the Allegheny or Appalachian mountains, have ascertained that the older fossiliferous rocks of that chain (from the Silurian to the Carboniferous inclusive) are not less than 42,000 feet thick, and if they were now superimposed on each other in the order in which they were thrown down, they ought to equal in height the Himalayas with the Alps piled upon them. Yet they rarely reach an altitude of 5000 feet, and their loftiest peaks are no more than 7000 feet high. The Carboniferous strata forming the highest member of the series, and containing beds of coal, can be shown to be of shallow-water origin, or even sometimes to have originated in swamps in the open air. But what is more surprising, the lowest part of this great Palæozoic series, instead of having been thrown down at the bottom of an abyss more than 40,000 feet deep, consists of sediment (the Potsdam sandstone), evidently spread out on the bottom of a shallow sea, on which ripple-marked sands were occasionally formed. This vast thickness of 40,000 feet is not obtained by adding together the maximum density attained by each formation in distant parts of the chain, but by measuring the successive groups as they are exposed in a very limited area, and where the denuded edges of the vertical strata forming the parallel folds alluded to at page 87 “crop out” at the surface. Our attention has been called by Mr. James Hall, Palæontologist of New York, to the fact that these Palæozoic rocks of the Appalachian chain, which are of such enormous density, where they are almost entirely of mechanical origin, thin out gradually as they are traced to the westward, where evidently the contemporaneous seas allowed organic rocks to be formed by corals, echinoderms, and encrinites in clearer water, and where, although the same successive periods are represented, the total mass of strata from the Silurian to the Carboniferous, instead of being 40,000 is only 4000 feet thick.