Upper Silurian.—At the close of the Lower Silurian, as already stated, changes of considerable importance took place in some of the areas which had undergone depression. The sea-bottom was now raised in parts of Western Europe, and in North America, and islands of some extent formed. In Europe the most important were those in North Wales, Shropshire, Cumberland, the South of Scotland, and in Nassau. It is probable also that one or more extended along Southern Europe, from Portugal into Spain, etc., but the evidence concerning this is as yet imperfect. In America, according to Logan, Dana, etc., the principal parts raised at this time were the Green Mountain regions, and the ridge extending from Lake Erie over Cincinnati into Tennessee. These American ridges appear to have been raised to a great height, and to have remained afterwards in part dry land, even to the close of the Palœozoic. The islands in Europe were raised only to a small height above water-level, and were probably all again submerged by the close of the Upper Silurian.