tainly where fossil mud-cracks penetrate a formation to the depth of 10 feet, as is the case in the Upper Shinarump (Triassic) shales of Utah, it is difficult to believe that they could be formed under other conditions than those permitting prolonged exposure such as is found only in the playas of the desert, where ten years or more may elapse between rainfalls. . . . . If the playa lake exists for some time it may become stocked with certain forms of organisms, especially types whose eggs or larvæ can be transported by wind or by birds. The small crustaceans Estheria, Daphnia, and Cypris are characteristic of desert lakes, the first being found in ponds which are dry for eleven successive months" (Grabau, 87, 707, 603). The nature of the organisms characteristic of such playa lakes is exceedingly interesting in view of the fact that Geikie adds to his description of the lithological characters of the beds in question th e following statement: "Fragments of fish and coprolite are scattered abundantly through most of the flagstones. Some of the calcareous shales are full of Estheria, while traces of plants occur in great numbers, though generally in a somewhat macerated condition" (71, 393). The close correspondence between the description of modern playa deposits and the Caithness flag portion of the Old Red Sandstone series leaves no reasonable doubt that the latter formation was the result of inland drainage in a semi-arid or desert region.
The detailed characteristics of a single series of beds in the Old Red have been taken as an example illustrating the conditions which prevailed, but attention need not be confined to any single part of the formation, for Goodchild has found evidence in all of the divisions of the Old Red to show that desert conditions prevailed throughout all the Devonic wherever this type of deposition obtained. In order not to burden the discussion with a too lengthy description of all of the features indicating desert or at least continental origin for these deposits I shall give a list setting forth the facts already cited and certain additional ones.
Summary of Evidence for Fluviatile Deposits. (a) Lithogenesis. (1) The presence of finely stratified, rippled and sun-cracked flags over an area of many square miles, and at successive horizons, the sun-cracks penetrating to a depth of five or six inches and being at times three or four inches wide, indicates playas or at least broad river flood plain conditions. These features have been noted by Geikie in the Thurso flags (71, 392, 393).
