CLIMATE AND TERRESTRIAL DEPOSITS 281 



of mud, which frequently is less extensive than the earlier deposits, thus giving 

 abundant opportunity for the observation of the exact manner of burial of the 

 older sand-covered stratum. The process described above is thought to be of 

 particular interest on account of the generality of its occurrence in this 

 desert.^ 



Ellsworth Huntington contributes the following statement con- 

 cerning relations observed by him in central Asia: 



The floor of the great desert basin of Lop in western China furnishes examples 

 of various associations of mud-cracks with aeolian deposits. 



At Yartungaz, 450 miles southwest of I^op-Nor, the floor of the basin consists 

 of a fine-grained saline silt which appears to have been deposited in broad, shallow 

 playas. The surface of the ground is smooth, and there is no sign of buckling, 

 but polygons from three to twelve feet in diameter are plainly visible. The division 

 lines stand out as dark markings dividing the light gray plain into innumerable 

 fairly symmetrical polygons. The prominence of the markings is due to the 

 fact that they are composed of sand which seems to have been blown by the 

 wind into the original cracks in the plain. 



A hundred mfles farther to the southeast, near Chira, the barrenness of the 

 wind-swept plain of piedmont gravel at the base of the Kwen Lun mountains is 

 slightly relieved by lines of grassy weeds arranged in polygonal patterns like 

 those of Yartungaz. Digging among them, one finds that the plants grow in 

 fine brown sand which fills old cracks in a hard deposit -of fine and very saline 

 gravel. The sand is of the same texture as that of the small dunes of the neigh- 

 borhood. 



The most striking case, however, is that of the old bed of the expanded glacial 

 lake of Lop-Nor. For scores of miles a layer of impure rock salt, from one to 

 three inches thick, has split into polygons ranging from five to twelve feet in 

 diameter. The edges have buckled up in exactly the same manner as the edges 

 of ordinary mud-cracks, and frequently stick up two or three feet. The under- 

 lying hollows are often partially filled with sand which has been blown in from 

 the distant shores of the lake-bed.^ 



Thus it is seen that this peculiar method of preserval of mud- 

 cracks is widely prevalent at the present time upon the flood-plains 

 of arid regions in various parts of the world. The writer has observed 

 that the same structure has been also widely developed in certain 

 ancient formations. 



In the Mauch Chunk shale of Pennsylvania the mud-cracked 

 shale partings between sandstone strata are frequently in the form 

 of tesselated polygonal plates concave upward, the covering layer of 



I Personal comraunication to the author. 



