282 STUDIES FOR STUDENTS 



sand filling the cracks and passing beneath the upraised rims of 

 the plates. ' 



In the Triassic of the Connecticut valley the same structure may 

 be frequently observed, the concave shale plates being in this forma- 

 tion frequently buried in undecomposed feldspathic sand. 



Second, interbedding of fluvial and aeolian sands. — Gravel and 

 sands which are originally transported by fluvial or pluvial actions 

 upon flood plains in arid climates are later subjected to the action of 

 the wind. Truly fluvial sands and gravels consequently become 

 repeatedly interbedded with aeolian sands, and Huntington states 

 that this is a striking and frequent relation, as seen in the freshly 

 exposed terrace faces on the margins of the desert basins of Asia. 



The fluvial and pluvial deposits are characterized by a more hetero- 

 geneous and coarser nature and by the absence of the characteristics 

 which mark aeolian sands. It is necessary therefore to summarize 

 some of the distinguishing features of the latter deposits. Dune 

 sands are deposited on the sloping surfaces of the leeward faces of 

 dunes and may reach heights of several hundred feet. At the bottom 

 the inchned layers decrease in dip and pass horizontally to the inter- 

 vening desert surface. In a region of accumulating sands each 

 passing dune leaves its basal portions to be covered by the march of 

 the following succession of dunes, so that the lower portion of this 

 cross-bedded structure becomes deeply buried and permanently 

 preserved. 



As has been recently pointed out by a number of geologists, the 

 characteristic features of such dune sands, separating them from fluvial 

 or littoral deposits, consist consequently in the homogeneous nature, 

 the development of "millet seed" texture, and the presence in striking 

 degree of cross-bedding which may reach great thicknesses. The 

 cross-bedded strata are abruptly truncated above but flatten out and 

 become tangejit to the general stratification at the bottom. It is 

 probable that a number of arenaceous formations which have been 

 customarily considered marine are, on the contrary, of such mixed 

 fluvial and aeolian origin. For example, Davis, Huntington, and 



I For illustrations see Joseph Barrell, "Origin and Significance of the Mauch 

 Chunk Shale," Bulletin 0} the Geological Society of America, Vol. XVITI (1907), Pis. 

 XLIX, L, and Fig. i, p. 457. 



