418 DONALD C. BARTON 



of arkose are intermediate between these extremes, and that 

 a moderately cool and arid climate such as would prevail at mod- 

 erately high altitudes in the lee of high mountain ranges or in con- 

 tinental interiors would more probably be suitable. The present 

 paper is an attempt to delimit the significance of arkose. 



The fundamental conditions essential for the formation of 

 arkose 1 are: (a) a granitic terrane, (b) conditions favorable to the 

 disintegration of the granite or gneiss with but slight accompanying 

 decomposition, and (c) conditions favorable to the erosion and 

 deposition of the debris of disintegration with merely slight loss 

 of the feldspar. In the investigation of the regions which today 

 can supply debris of disintegration for the formation of arkose 

 (Figs, i and 2), it was found that disintegration is much more 

 widespread than is perhaps usually realized, and that it takes 

 place in marked amounts under practically all the conditions under 

 which a granitic terrane is exposed (see Table I, a list of the occur- 

 rences of disintegration which have been observed by the writer, 

 or which he has been able to find described in the literature, together 

 with a tabular view of the conditions under which the disintegration 

 is taking place). The investigation of the conditions under which 

 the disintegrated material could be eroded and deposited as arkose 

 seems to show that in some cases the conditions favorable to the 

 disintegration are likewise favorable to contemporaneous erosion 

 and deposition of the disintegrated material as arkose, as, for 

 instance, in desert regions, and that in other cases erosion can take 

 place only after some change of conditions, as, for example, a 

 change from the conditions of a moist temperate climate to those 

 of a semi-arid climate. In yet other cases, erosion may take place 

 contemporaneously with disintegration but be followed by decom- 



1 Arkose by original definition and according to most general usage is a rock 

 formed of the relatively undecomposed debris of granite or of rods of granitic mineral- 

 ogical composition. It may be thought, however, that the original definition should 

 be extended to cover feldspathic elastics derived from the disintegration of syenites, 

 diorites, gabbros. Feldspathic elastics of this type, however, should be rare, since 

 there are practically no purely syenitic, dioritic, or gabbroic terranes, and since the 

 plagioclase feldspar of the diorites and gabbros is more or less readily decomposed. 

 No specimen of this type of feldspathic clastic has been seen by the writer, and only 

 one or two reputed occurrences are reported in the literature. 



