CLASSIFICATION OF ARKOSE DEPOSITS 435 



A. ARKOSE DEPOSITS FORMED ENTIRELY UNDER RIGOROUS 

 CLIMATIC CONDITIONS 



Feldspar showing merely slight decomposition. 1 Argillaceous 

 material absent or present in minor amounts. 2 



1. Deposits formed in desert regions. — The deposits are massive, 

 homogeneous, and in some cases of very large size. In desert 

 regions, the disintegration takes place chiefly on outcrops directly 

 exposed and not protected, as in moist temperate regions, by a 

 mantle of vegetation. The debris of disintegration is easily 

 eroded and the processes of erosion and disintegration and deposi- 

 tion of the eroded, disintegrated material as arkose can therefore 

 take place contemporaneously, and can continue as long as the 

 desert conditions persist and as long as the granitic terrane remains 

 unburied. The size of an arkose deposit formed under such con- 

 ditions will therefore depend chiefly on the size of the terrane of 

 disintegration, the size of the basin of deposition, and the length 

 of time the desert conditions prevail. The constancy of the condi- 

 tions during the period of formation should be marked by a massive- 

 ness and homogeneity of the deposits. 



a) Terrestrial: In deserts, wind action prevails the greater part 

 of the time, but rare storms do occur and in the short space of their 

 existence do an immense amount of work. Deposits of arkose 

 formed in desert regions therefore are likely to be in part of eolian 

 and in part of aqueous origin. The arkose shows in part the 

 eolian characteristics of well-rounded sand grains, faceted pebbles, 

 local lag-gravels, dune stratification, etc., and in part the ordinary 

 characteristics of water action. In deposits of arkose forming in 

 arid mountain regions, the greater part of the transportation of 

 the disintegrated material may be by water action, during the rare 

 cloudbursts, and by the streaming of the debris down the hill slopes 

 under gravity. The constituent grains in this case are subangular, 

 and quartz and feldspar grains should be present in about the same 

 proportions as in the original granite or gneiss, since the amount of 



1 At the time of formation of the arkose. Subsequent exposure may produce 

 complete decomposition. 



2 The possibility of the presence of exotic material brought in by a river whose 

 headwaters lie in a temperate or tropical region should not be forgotten. 



