Page:The Habitat of the Eurypterida.djvu/196

188 no doubt, be regarded as portions of the upper muddy layer which cracked off and curled up during desiccation, as may often be observed on dried-up pools at the present time. Some pittings, occasionally seen on the sun-cracked surfaces, may perhaps represent rain-drops" (71, 392, 393).

Such characteristics as those just cited have been used by Geikie as proof of the lake shore origin of the beds and by other writers as indicative of their formation in mud-flats along the sea coast. Were it not that such interpretations are offered by the majority of geologists it would be unnecessary to dwell upon the unequivocal interior continental origin of these features. That mud-cracks should be formed over wide areas indicates beyond a doubt the presence of a large body of very shallow water which completely evaporated, leaving the whole surface exposed to the air. Not only that, but the exposure must have been long for the cracks to be 5 or 6 inches deep and occasionally 3 or 4 inches wide. Professors Grabau and Barrell have discussed this subject of ripple marks and sun-cracks over wide areas in such a convincing and logical manner that it need not be taken up in detail here. In his Principles of Stratigraphy Professor Grabau cites the case of the great playa in the Black Rock Desert, Nevada, which forms in a few minutes and covers an area of from 450 to 500 square miles and yet is seldom over a few inches in depth. Russell has described this lake and records that in a few days all of the water may dry up leaving the surface cracked in all directions. "The lake beds then have a striking resemblance to tesselated pavements. . . . . "—the very words used by Geikie in describing the Old Red flagstones! Grabau says: "Taking the areas of mud-crack formation in the order of their magnitude, the playa surface would probably stand first. Here the entire surface for hundreds of square miles becomes mud-cracked, often to considerable depth, on the complete drying up of the temporary playa lake. Here, too, the conditions for the preservation are most favorable. Not only is the exposure a long one, often the greater part of the year, or for many years, and for much of the time to intense heat, but the chances of proper burial are much greater. Wandering sand dunes may thus preserve the record, dust deposits may fill the fissures, or, at the next flood, sands or muds may be swept into them. In fact, the playa or takyr seems to be the ideal surface for mud-crack record, and one is tempted to refer most mud-cracked strata to such an origin.