This vast formation of secondary, extending as it probably does from the Gulf of Mexico to the Northern ocean, and from the Bay of St. Lawrence {322} to the Rocky Mountains, must of necessity occupy in various parts different and sometimes great elevations: like other great fields of the same formation, its borders are marked by high and broken ridges, which become less elevated and less frequent towards the centre. Sandstone appears to be the basis and predominating rock occupying the borders contiguous to the primitive and transition, and passing under the more recent secondary. In this sandstone on the outskirts of the secondary, have been found most of the extensive coal beds hitherto known, also gypsum and brine springs.
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.
1st. Granular Limestone—Appears in every part of the United States, where it has hitherto been observed to be the uppermost in the series of primitive rocks. It is true, it is often found to graduate, by minute and imperceptible shades of difference, into that which is decidedly secondary. Instances of this have been observed so frequently that the fact can be no longer questioned. This fact, and others of the same kind, ought not, perhaps, to be considered as invalidating the received opinions with regard to the classification of rocks according to the doctrines of Werner. If a division is to be made of the rocky strata of the earth into primitive, transition, &c. it is, perhaps, of little importance whether the boundaries thus instituted shall traverse beds of the same substance, {324} or separate contiguous strata composed of different materials.
That series of rocks next in order to the primitive limestone above mentioned, has been very generally denominated the Transition Class. It comprehends the following strata: Metalliferous limestone, Clay-slate, Graywacke, and Graywacke-slate, and Old Red sandstone. If we confine our attention to the consideration of these rocks as they exist in our own country, we shall find them appearing in their different localities under circumstances of considerable uniformity.
2d. Metalliferous Limestone.—The prevailing colour of this rock is blue, of various shades and intensities, varying into yellow and gray. It has usually a close texture, an even, large conchoidal, or somewhat splintery fracture. In many varieties the surface, by long exposure, becomes coated with an incrustation of a yellowish white powdery matter, which adheres closely. It is frequently traversed by small reticulating veins of quartz or calcareous spar, which, during the gradual decomposition of detached masses, resist the progress of disintegration, and are left standing out from the surface, giving it a chequered appearance. It is the lowest and is considered as the most ancient of the rocks containing organized remains, which are those of cryptogamous plants, and animals without sight.
Geographical distribution.—This rock occurs extensively along all the north-western side of the primitive of the great chain of the Alleghanies. In lower Canada and Vermont, it is accompanied by granular limestone and granular quartz, which separate it from the mica slate and talcose rocks on the east. [See Eaton's Index to the Geology of the Northern States.] It is there usually inclined towards the west, at an inconsiderable angle. It is separated from the compact fletz limestone of the valley of Lake Champlain by a stratum of old red sandstone, which forms {325} the upper part of a range of hills, called, in Vermont, the Snake mountain. In Berkshire county, in the western part of Massachusetts, and along the eastern side of the Hudson in New York, a stratum of primitive clay-slate intervenes between this rock and the granular limestone. The New Lebanon mountain, which is of slate, and divides the primitive limestone of Pittsfield, Richmond, Stockbridge, &c. from the transition which occurs at New Lebanon springs, and along the western base of this range, is considered primitive. (Dewey in Silliman's Journal.) To the north-east of the Hudson river, the transition limestone nowhere occupies any great extent of surface from east to west, but is a narrow strip running along the margin of the primitive, and in a few miles is succeeded either by red sandstone, or clay-slate resting upon it. In Vermont, in the same neighbourhood, it alternates with clay-slate, and supports red sandstone.