BUFFALO SOCIETY OF NATURAL SCIENCES 189 



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 con- 

 ditions 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, espe- 

 cially types whose eggs or larvae 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 exceed- 

 ingly interesting in view of the fact that Geikie adds to his description 

 of the lithological characters of the beds in question the 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). 



