CLASSIFICATION OF ARKOSE DEPOSITS 421 



position of the feldspar, with the consequent loss of the material 

 as a possible source of arkose. It was therefore found impossible 

 to limit the significance of arkose to significance of any one or two 

 special sets of conditions. The arkose deposits forming under the 

 different conditions should expectedly be of certain characteristic 

 types, which as a matter of fact agree with the types of arkose 

 deposits as they are found (see Table II) . The genetic classification 

 of arkose deposits which appears in the following pages is therefore 

 intended to embrace these various types of arkose deposits. 



GENETIC CLASSIFICATION OF ARKOSE DEPOSITS 



Arkose deposits may be divided broadly into two classes: 

 (a) those formed directly through the effects of rigorous climatic 

 conditions; and (b) those formed, at least indirectly, through the 

 effects of moist and more temperate climatic conditions. The latter 

 conditions allow much decomposition, which, however, commonly 

 takes place at a slower rate than the disintegration. The arkose 

 formed directly or indirectly under these conditions therefore has 

 feldspars showing considerable decomposition, has in many cases 

 a matrix of argillaceous material derived from the more easily 

 decomposed grains of feldspar and other silicates, and is associated 

 with beds of argillaceous material derived from the totally decom- 

 posed portions of the rock. The former conditions are unfavorable 

 to decomposition, and the arkose deposits forming under them have 

 comparatively unaltered feldspars, have little or no argillaceous 

 matrix, and are not associated to any great extent with argillaceous 

 beds. The distinction, however, is not absolute. At Aswan, 

 Egypt, in a region in which no rain is recorded over a period of 

 many years, the disintegrated granite in some places shows marked 

 decomposition, and the feldspar of modern arkose deposits in the 

 beds of the wet-weather streams of the region is deeply decomposed. 

 In regions of moist, temperate climate it is not uncommon, on the 

 other hand, to find below the zone of complete disintegration a 

 zone of rock which to the eye seems fresh but which crumbles 

 readily under blows from the hammer, and from this rock, within 

 a region of moist, temperate climate, it would be possible for arkose 

 with relatively fresh feldspar to form. 



