The breadth of the transition district, like that of the primitive, is variable. Narrow towards the gulf of Mexico, it gradually widens towards the north-east, till it reaches the river Hudson. From its upper portion it sends off a considerable arm, which penetrates for several hundred miles into the granitic region, overlaying it, but running parallel with the principal body. After the primitive, it forms some of the highest mountains in the range, and seems to be both higher and wider to the west in Pennsylvania, Maryland and part of Virginia, where the primitive is least extended and lowest in height. It contains all the varieties of rocks found in the same formation in Europe. It varies in breadth from twenty to one hundred miles. In the limestone of this formation there are many and extensive caves, some of which extend for miles under ground, and contain the bones of animals. It is the lowest, and is considered the most ancient of the rocks containing organized remains, which are those of cryptogamous plants, and animals without sight. The graywacke has been observed to contain impressions of organized remains, but they are usually those of zoophytic animals, and are exceedingly unlike those found so abundantly in the coal formations. Its colors are variable; it is, however, most commonly bluish, black, or dark brown. The graywacke seems to form the connecting link between the clay-slate and a rock which has been called the old red sandstone, and is usually found intimately blended either with the one or the other. This sandstone occurs throughout the whole extent of the transition formation, and evidently belongs to the oldest depositions of that rock. It is for the most part distinctly stratified, and in all cases its stratification is inclined. Of the rocks thus described, the limestone occurs extensively all along the north-western side of the primitive strata. It is probable that transition limestone is the foundation through their whole extent of the Alleghany mountains of Pennsylvania, Maryland and the western parts of Virginia, on a level with the surface at the base of their eastern declivities. The clay-slate occurs in the central portions of that extensive field of transition, which skirts the western margin of the primitive of New York and New England, and forms the great body of the Catskill mountains. The old red sandstone in the transition district, along the whole range of mountains, is perhaps more abundant than any other aggregate. This region has also a considerable mixture of trap. Various large bodies of transition rock are thrown to a considerable distance into the primitive region; while in many instances, secondary rocks are found running along the valleys far into the bosom of the mountains.