Page:The Habitat of the Eurypterida.djvu/195

Rh and even the water which was collected into streams would be lost by evaporation or by sinking into the ground. Great alluvial fans were spread out, consisting of coarse conglomerates near the source of supply and of sands farther away. During those periods when the infrequent but heavy rains fell, playa lakes undoubtedly were formed, similar to those known to be characteristic in present semiarid regions which have periodically inundated river flood plains. Evidence is not wanting that just such water bodies did form, for Geikie has called attention to certain characteristics in the Thurso flags which admit of no other interpretation. Along the northern coast of Caithness from Castletown to Thurso, a distance of some seven miles along the beach, these flagstones are exposed in great sheets. They consist of "fissile, calcareous, grey, hard flagstones, green, gray and brown calcareous (and frequently bituminous) shales, with thin bands of calcareous gritty sandstone and argillaceous limestone ('calmy limestone'), seldom more than a few inches in thickness. . . . . Even when split into smooth sheets an inch or less in thickness, these hard, tough layers show on their yellow, weathered edges successive paper-like but mutually adherent laminae. . . . . "

A second feature is "the extraordinary abundance of ripplemarked surfaces and sun-cracks. Though these markings abound also in the lower flagstone group, it is here that they attain their greatest development. Surfaces of flagstone or shale, many square yards in extent, are profusely covered with fine ripple lines as sharply preserved as if only today imprinted on the soft sediment. In many places every successive stratum or leaf of rock is thus marked, so that several distinct rippled surfaces may be counted in the thickness of a few inches of rock. It is likewise observable that the rippling is generally close-set, sometimes not exceeding an inch in breadth from crest to crest of the ridges."

Mud-cracks form a third important structure. Geikie says: "More abundant and admirable illustrations of sun-cracks could hardly be found than occur along the coast. Broad, gently-inclined sheets of rock again and again present themselves to view so covered with reticulations as to look like tessellated pavements. It may be noticed that the cracks not infrequently descend through many of the fine laminæ of deposit for a depth of 5 or 6 inches with occasionally a breadth of 3 or 4 inches. The material filling up the interstices abounds with small, occasionally curved pieces of shale. These may,