This plateau, the third great band of the Appalachians, begins on the south side of the Mohawk Valley. To the north its place is taken by the Adirondacks, which are an outlier of the great Laurentian area of Canada. The fact that the outlier and the plateau are separated by the low strip of the Mohawk Valley makes this the one place where the highly complex Appalachian system can easily be crossed. If the Alleghany plateau joined the Adirondacks, Philadelphia instead of New York would be the greatest city of America. Where the plateau first rises on the south side of the Mohawk, it attains heights of four thousand feet in the Catskill Mountains. We think of the Catskills as mountains, but their steep cliffs and table-topped heights show that they are really the remnants of a plateau, the nearly horizontal strata of which have not yet been worn away. Westward from the Catskills the plateau continues through central New York to western Pennsylvania. Those who have traveled on the Pennsylvania Railroad may remember how the railroad climbs the escarpment at Altoona. Farther east the train has passed alternately through gorges cut in the parallel ridges and through fertile open valleys forming the main floor of the inner valley. Then it winds up the long ascent of the Alleghany front in a splendid horseshoe curve. At the top, after a short tunnel, the train emerges in a wholly different country. The valleys are without order or system. They wind this way and that. The hills are not long ridges but isolated bits left between the winding valleys. Here and there beds of coal blacken the surface, for here we are among the rocks from which the world's largest coal supply is derived. Since the layers lie horizontally and have never been compressed, the same material which in the inner valley has been changed to hard, clean-burning anthracite here remains soft and smoky.