1 88 THE HABITAT OF THE EURYPTERIDA 



no doubt, be regarded as portions of the upper muddy layer which 

 cracked off and curled up during desiccation, as may often be ob- 

 served on dried-up pools at the present time. Some pittings, occa- 

 sionally 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-fiats along the sea coast. Were 

 it not that such interpretations are offered by the majority of geolo- 

 gists 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, leav- 

 ing 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 for- 

 mation 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 com- 

 plete drying up of the temporary playa lake. Here, too, the condi- 

 tions 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. Cer- 



