The Devonian age was the age of fishes or vertebrates, and owing to the need of greater elasticity of their bones and smaller weight, they are composed of phosphate of calcium, or “lime.” Far back in the Silurian age the “Hand that fashioned all things well” began to change some of these shell fish to provide for the future order of things, so that their outside skeletons, or shells, were composed of phosphate of lime instead of carbonate. These two commingling, the resultant beds of rock became somewhat phosphatic and formed the “phosphatic limestones” of the Silurian age. In some places the phosphate shells were in considerable proportion, and subsequent erosion, proper underground drainage and leaching, dissolved out the carbonate of lime to greater or less extent and left the “brown phosphate” of the middle basin, varying in grade according to the preponderance of phosphate shells in the original deposit and the extent of the subsequent leaching. Meantime the transition stage between the two ages had been reached and the resulting deposit spread over the central basin and the highland rim in the form of a thin blanket of varying thickness and quality of the so-called blue rock, which is blue, brown, gray and black, according to the coloring matter present or absent, composed of a preponderance of microscopic shell fish with skeleton composed of phosphate of lime, but mixed with enough carbonate to make the resulting mass vary from sixty-five per cent to as high as eighty per cent calcium phosphate.
The subsequent depression and deposit of Devonian shales and subcarboniferous beds and subsequent great pressure hardened all these into rock, and about the middle of the subcarboniferous age all these were elevated above the surrounding country, and while the rest of the land was taking its turn in being formed under the seas, this old central basin was undergoing the wear and tear of erosion that finally produced the “Dimple of the Universe,” surrounded by its chain of hills and ridges and flatwoods of the highland rim.
In the central basin where conditions were favorable the intervening strata between the blue rock and the phosphatic limestones that were being converted into “brown rock” were sometimes partly and sometimes entirely washed away, and the blanket of the blue rock, cracked and broken into plates of the hardest and most durable parts, settled down on the brown rock, sometimes resting directly on it and sometimes with a clay seam left to represent the former intervening strata.
A glance at the illustrations will show the process. Let Figure A represent the deposit as it originally was before the erosion and leaching started in.
No. 1 represents the layer of blue rock in its original position; 2 the layers of limestone underneath; 3 the layers of highly phosphatic limestone in suitable condition for leaching; 4 the hard, insoluble portions of the limestone, and 5 the soluble portions of the limestone, nonphosphatic.
In Figure B, No. 5 has dissolved out and disappeared. In No. 3, the carbonate has leached out and it has separated into laminations and falling into the places left by No. 5, has assumed the jumbled condition found in the dips between lime boulders; No. 2 has dissolved down to the clay seam so generally found, varying in thickness from one or two inches to two or three feet, and No. 1 has settled down conforming to the general bottom of 3 or top of 4, forming the top rock generally prevalent in the brown rock field.
If the analysis of the original phosphatic limestone was, say, 50 per cent phosphate of lime, 38 per cent carbonate and 12 per cent insoluble, and other matters, and the leaching took out all the soluble carbonate, the resulting mass would be 80 per cent phosphate, which is generally the analysis of the bottom rock at Mt. Pleasant, or the “export,” as it is termed. The top rock varies in analysis from 65 to 80, just as the original blue rock did.
In the highland rim this process took place only on the slopes of the narrow creek valleys and occasionally in projecting points, instead of over large areas of country as in the central basin.
In the portion of the highland rim left intact the blue rock remains in place as a general thing with its varying quality and thickness, retaining its original compact form and density.