Note BC — Chapter XIII

Some of the clay-slates of the primary formations abound in these circular, uncolored patches, bearing in their centres, like the patches of the Old Red Sandstone, half obliterated nuclei of black. Were they, too, once fossiliferous? and do these blank erasures remain to testify to the fact? I find the organic origin of the patches in the Old Red Sandstone remarked by Professor Fleming as early as the year 1830, and the remark reiterated by Dr. Anderson, of Newburgh, in nearly the same words, but with no acknowledgment, ten years later. The following is the minute and singularly faithful description of the Professor:—

"On the surface of the strata in the lower beds, circular spots, nearly a foot in diameter, may be readily perceived by their pale yellow colors, contrasted with the dark red of the surrounding rock. These spots, however, are not, as may at first be supposed, mere superficial films, but derive their circular form from a colored sphere to which they belong. This sphere is not to be distinguished from the rest of the bed by any difference in mechanical structure, but merely by the absence of much of that oxide of iron with which the other portion of the mass is charged. The circumference of this colored sphere is usually well defined; and at its centre may always be observed matter of a darker color, in some cases disposed in concentric layers, in others of calcareous and crystalline matter, the remains probably of some vegetable or animal organism, the decomposition of which exercised a limited influence on the coloring matter of the surrounding rock. In some cases I have observed these spheres slightly compressed at opposite sides, in a direction parallel with the plane of stratification—the result, without doubt, of the subsidence or contraction of the mass, after the central matter or nucleus had ceased to exercise its influence."—(Cheek's Edinburgh Journal, Feb. 1831, p. 82.)