VALLEYS IN LOW LATITUDES I2I 



many feet of disintegrated material were found above the more 

 indurated granite into which the arkose graded slowly. This granite 

 in its upper portion is soft and partially disintegrated. In its lower 

 portion certain "bowlders" or less disintegrated portions in it were 

 being quarried for building purposes. It was not altogether clear 

 whether the "bowlders" are to be regarded merely as residual rem- 

 nants of a once homogeneous mass left by the unequal progress of 

 disintegration, or whether the whole was a secondary deposit of mixed 

 arkose and bowlders derived from the adjacent hills and once partially 

 cemented together, but which has more recently again resumed its 

 disintegration. There are some reasons for believing that both alter- 

 natives are true in different portions of the district. In those localities 

 where deep disintegration prevails, the upper slopes merge into the 

 lower slopes in the gradual, curving way common in higher latitudes. 

 The main purpose of these notes is to lay emphasis on the relatively 

 sharp angle between the valley sides and the valley bottoms observed 

 in several of these regions of low latitude. In part this feature may, 

 no doubt, be said to be due to the comparative absence of talus at the 

 foot-slope of the valley sides, but this is probably not the whole truth 

 of the matter. The relative absence of coarse talus of a certain 

 kind in low latitudes is recognized as assignable to the absence of 

 freezing temperatures by virtue of which the repeated expansion 

 of water in joint planes, cracks, and pores disrupts the surface 

 rocks and loosens masses which roll to the foot of the slope.' The 

 localities named in this paper, except that in . Japan, are all 

 within the border of the tropics and this explanation is applicable 

 so far as it goes. Talus piles and scree slopes of the frost type are 

 obviously excluded by the climatic conditions. Bowlders due to 

 exfoliation, which we found so abundant at Kowloon, are probably 

 absent in the localities characterized by thin soils because in these 

 cases the decay proceeded more uniformly and slowly from the sur- 

 face and was more largely confined to the surface instead of penetrat- 

 ing deeply along the joints and working from them toward the centers 

 of the blocks between the joints. This limitation of action was prob- 

 ably due in turn to the close texture of the rock in the Mexican, Oahu, 



I J. C. Branner, "Decomposition of Rocks in Brazil/' Bull. G. S. A., Vol. VII, 

 1856, p. 268. 



