Fig. 10.—Carboniferous rill-mark (Nova Scotia), reduced, to illustrate pretended Algæ.
Shrinkage-cracks are also abundant in some of the Carboniferous beds, and are sometimes accompanied with impressions of rain-drops. When finely reticulated they might be mistaken for the venation of leaves, and, when complicated with little rill-marks tributary to their sides, they precisely resemble the Dictyolites of Hall from the Medina sandstone ([Fig. 11]).
Fig. 11.—Cast of shrinkage cracks (Carboniferous, Nova Scotia), illustrating pretended Algæ.
An entirely different kind of shrinkage-crack is that which occurs in certain carbonised and flattened plants, and which sometimes communicates to them a marvellous resemblance to the netted under surface of an exogenous leaf. Flattened stems of plants and layers of cortical matter, when carbonised, shrink in such a manner as to produce minute reticulated cracks. These become filled with mineral matter before the coaly substance has been completely consolidated. A further compression occurs, causing the coaly substance to collapse, leaving the little veins of harder mineral matter projecting. These impress their form upon the clay or shale above and below, and thus when the mass is broken open we have a carbonaceous film or thin layer covered with a network of raised lines, and corresponding minute depressed lines on the shale in contact with it. The reticulations are generally irregular, but sometimes they very closely resemble the veins of a reticulately veined leaf. One of the most curious specimens in my possession was collected by Mr. Elder in the Lower Carboniferous of Horton Bluff. The little veins which form the projecting network are in this case white calcite; but at the surface their projecting edges are blackened with a carbonaceous film.
Slickensided bodies, resembling the fossil fruits described by Geinitz as Gulielmites, and the objects believed by Fleming and Carruthers[Y] to be casts of cavities filled with fluid, abound in the shales of the Carboniferous and Devonian. They are, no doubt, in most cases the results of the pressure and consolidation of the clay around small solid bodies, whether organic, fragmentary, or concretionary. They are, in short, local slickensides precisely similar to those found so plentifully in the coal under-clays, and which, as I have elsewhere[Z] shown, resulted from the internal giving way and slipping of the mass as the roots of Stigmaria decayed within it. Most collectors of fossil plants in the older formations must, I presume, be familiar with appearances of this kind in connection with small stems, petioles, fragments of wood, and carpolites. I have in my collection petioles of ferns and fruits of the genus Trigonocarpum partially slickensided in this way, and which if wholly covered by this kind of marking could scarcely have been recognised. I have figured bodies of this kind in my report on the Devonian and Upper Silurian plants of Canada, believing them, owing to their carbonaceous covering, to be probably slickensided fruits, though of uncertain nature. In every case I think these bodies must have had a solid nucleus of some sort, as the severe pressure implied in slickensiding is quite incompatible with a mere “fluid-cavity,” even supposing this to have existed.
[Y] “Journal of the Geological Society,” June, 1871.
[Z] Ibid., vol. x., p. 14.