190 THE HABITAT OF THE EURYPTERIDA 



(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 playa or a river flood plain. 



(3) The basal conglomerate of the Orcadian series has character- 

 istics pointing to the fact that it is made up of materia] derived from 

 the disintegrated but not decomposed underlying rocks, thus indi- 

 cating dry climatic conditions during its formation. The conglomer- 

 ate 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 under- 

 lying 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 beauti- 

 fully 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 in- 

 stance 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 visi- 

 ble. Here "the matrix, red in colour, and less strongly felspathic 

 than towards the south, contains large and usually rather well water- 

 worn 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. 



