Horizontally stratified limestone is met with in many parts of this formation, but is most abundant in the central portions, about the beds of the great rivers, and in those parts which have the least positive elevation. Compact limestone is a name sometimes used to designate all the varieties of that rock occurring in districts of secondary, but is certainly inapplicable to the limestone about Cape Girardeau and in many other places, which is notwithstanding manifestly secondary. Some of the limestone north-west of the primitive on Hudson's river, about the Coatskill and Hellebergh mountains, is of this crystalline variety, but abounds in marine exuviæ. That of Lake Champlain, as well as the greater part of that in the interior and western parts of the state of New York, is of the compact blue variety. From the falls of the Ohio at Louisville to Cincinnati, a mixed kind, partaking of the character of both of the before-mentioned varieties, is found along the river, and for some distance on each side. From Dr. Drake we learn, that this limestone is confined to a small district, and is on all sides bounded by sandstone, which rises from below it, and on which it is supposed invariably to rest. Whether the red sand-rock which is found on the south-west branches of the Arkansa, in a horizontal position, and in an highly inclined one skirts the Rocky Mountains, {323} extends to other parts of this formation of secondary, we are unable to say. Throughout the country adjacent to the Ohio river, the prevailing and basis rock is a gray horizontal sandstone, often approaching in character those varieties which contain coal. It embraces extensive beds of coarse conglomerate, and supports or alternates with compact limestone. _Of the Alleghany Mountains_ By this name we intend to designate the great range of mountains extending parallel to the Atlantic coast, from the sources of the St. John's river in New Brunswick in the north-east, to the confluence of the Alabama and Tombigbee in the south-west. An outline of this great chain has already been traced by Maclure, and particular accounts of portions of it are to be found in the works of Eaton and others; we shall, therefore, confine our attention to those strata, which, forming the north-western side of the range, are most intimately connected with the great secondary formations of the west.