BUFFALO SOCIETY OF NATURAL SCIENCES 187 



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 semi- 

 arid 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 lime- 

 stone ('calmy limestone'), seldom more than a few inches in thick- 

 ness 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 lami- 

 nae " 



A second feature is "the extraordinary abundance of ripple - 

 marked 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 laminae 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, 



