Another kind of markings not even organic, but altogether depending on physical causes, are the beautiful branching rill-marks produced by the oozing of water out of mud and sand-banks left by the tide, and which sometimes cover great surfaces with the most elaborate tracery, on the modern tidal shores as well as in some of the most ancient rocks. _Dendrophycus_[V] of Lesquereux seems to be an example of rill-mark, as well as _Aristophycus_, _Cloephycus_, and _Zygopliycus_, of Miller and Dyer, from the Lower Silurian.
[V] "Coal Flora of Pennsylvania," vol. iii., Plate 88.
Rill-marks occur in very old rocks,[W] but are perhaps most beautifully preserved in the Carboniferous shales and argillaceous sandstones, and even more elaborately on the modern mud-banks of the Bay of Fundy.[X] Some of these simulate ferns and fronds of Laminariæ, and others resemble roots, fucoids allied to _Buthotrephis_, or the radiating worm-burrows already referred to (Fig. 10).
[W] "Journal of the Geological Society," vol. xii., p. 251.
[Illustration: 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).
[Illustration: 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.
