Page:The Habitat of the Eurypterida.djvu/198

190 (2) The presence of clay galls in the deep interstices between the sun-cracked prismatic layers in the Thurso flags indicates exposure of clayey surfaces to the air long enough for flakes to be curled up and blown into the cracks. Such a feature might characterize any sun-cracked area, but the depth of the cracks as cited in (1) indicates a play a or a river flood plain.

(3) The basal conglomerate of the Orcadian series has characteristics pointing to the fact that it is made up of material derived from the disintegrated but not decomposed underlying rocks, thus indicating dry climatic conditions during its formation. The conglomerate is too thick to represent the basal conglomerate formed by an advancing sea, even if other characteristics did not preclude the marine origin. In detail the characteristics are as follows: (a) "The blocks vary in size up to as much as a yard, or even more, in length, and consist of gneiss, pink granite, quartz-porphyry, quartz-rock, mica-schist, and other crystalline rocks, with abundance of pink cleavable orthoclase derived from the underlying gneiss" (71, 375). In the Caledonian series the blocks are even larger, Hickling having recorded them up to 8 feet in diameter. (b) In every case the underlying rock from which the conglomerate boulders were derived can be found not far away. "Near the granite they (the boulders) are made up in great measure of granitic debris. Round the quartz rock they are largely composed of that material. The existence of the well-veined orthoclase gneiss is indicated some distance before the underlying rock is actually seen by the abundant fragments of beautifully cleavable pink felspar in the conglomerates" (71, 370). (c) In both of the quotations just given reference is made to the abundant presence of fresh pink orthoclase. Goodchild has likewise referred to the arkoses with unweathered feldspar fragments (80, 219), and has pointed out that they indicate disintegration under semi-arid or desert conditions. (d) The basal conglomerate is too thick to be of any other than fluviatile, more especially torrential origin. For instance at Sarclet, about five miles south of Wick, Caithness, a great mass, 250 to 300 feet high, rises from the sea, the base not being visible. Here "the matrix, red in colour, and less strongly felspathic than towards the south, contains large and usually rather well waterworn fragments of quartz-rock, granite, felspar, porphyry, and red sandstone" (71, 376). On no sea or lake beach is a large boulder conglomerate 250 feet thick ever formed by the action of waves.