ALLEGHANY COAL-FIELD IN ALABAMA.
That portion of the Alleghany coal-field in Alabama covers 4300 square miles of superficial area, and forms the southern extremity of the field. Its form is peculiar, as shown by the accompanying map, which is con densed, with some alterations, from the Alabama State Survey by Prof: Tuomey, and forms the knob on the handle of the great club which the Alleghany coal-field represents on paper, as the map of Italy takes the form of a boot under the same representation.
The end of the formation proper would appear to terminate with the high terminal points of the Lookout and Sand Mountains ; but the evidence is unmistakable of a sudden depression of the great mountain ranges and a corresponding depression of the superincumbent coal strata. The connection has also been found to be almost continuous between the coal measures, and the accompanying millstone grit and Carboniferous limestones occupy, geologically, the same horizon, or order of stratification, which we find general in this coal-field.
We might be led to suppose, from Professor Tuomey's report, that the limestones and coal measures were stratified unconformably upon the Silurian formations; but such is not the case. We investigated this subject fully during a late visit to Alabama, and found the same general contrac tion prevalent in the Eastern Paleozoic and Azoic formations which we find so general on the eastern border of the great basin. The eastern strata
are here folded sharply against the gigantic sides of the Lookout Mountain; but the strata in that mountain-range and that on which they rest are con formable to the coal measures, and not unconformable, as represented in Taylor's statistics, and as copied from Professor Tuomey's reports.
The Coosa, Cahawba, or St. Clair coal-field, as it has been variously called, seems to be part of the same great field. But this portion has been depressed to a greater extent than the more western basins, and slightly folded in basin-shape by the lateral contraction which folded the parallel Silurian strata to the east.
The gradual thinning or depreciation of the Palaeozoic strata in this direction, though not so rapid as it is westward from the point of its greatest thickness in Pennsylvania, has still so diminished the strata inter vening between the coal and the Great Valley limestone No. 11, that the Carboniferous lime and this—the Auroral lime—are brought almost into contact, compared with their vast distance apart in the northeast.