44© DONALD C. BARTON 



subsoil is much larger in proportion to the zone of disintegrated 

 material. The rate of disintegration under the conditions of a moist 

 temperate climate seems to be rather slow — in New England there 

 has been since the Glacial Period disintegration sufficient barely 

 to efface the glacial striae and polish on granitic and gneissic 

 ledges — and the very considerable amounts of disintegrated material 

 generally found in those regions are the result of slow accumula- 

 tion under the protection of the mantle of vegetation. General 

 erosion of this disintegrated material and its subsequent deposition 

 as arkose can take place only when the mantle of vegetation is 

 critically weakened or destroyed. When this has once happened 

 and the mantle of disintegrated material has been swept away, a 

 long time must elapse before considerable amounts of the disin- 

 tegrated material can again accumulate. The arkose deposits 

 formed from the accumulated debris of disintegration in a moist 

 temperate climate will therefore be of small or moderate size. As 

 the mantle of soil and completely decomposed rock is eroded at 

 the same time as the mantle of disintegrated material, the arkose 

 is commonly associated with mudstones and shales, and, as 

 the disintegration is accompanied by considerable decomposition, 

 the arkose itself is likely to contain much argillaceous material 

 and to have feldspars showing noticeable decomposition. Since 

 stream transportation of debris results in the rather rapid elimina- 

 tion of the feldspars, the arkose is likely to grade into quartzite. 



The causes which might critically weaken or overcome the 

 mantle of vegetation and result in the erosion of the accumulated 

 products of disintegration are: introduction of arid or semi-arid 

 conditions, introduction of subglacial or glacial conditions, a marked 

 increase of rainfall, a marine transgression, deforestation by forest 

 lire, and marked upwarping. A marked change of climatic con- 

 ditions toward aridity in a region previously of moist temperate 

 climate would necessarily result in a marked diminution of the 

 vegetation and in the exposure of the underlying disintegrated 

 material to erosion during the occasional storms. Glacial condi- 

 tions might result either in the erosion of the disintegrated material 

 by the ice itself or in the exposure of the disintegrated material 

 to erosion through the destruction of the vegetation of the temperate 



