Il8 T. C. CHAMBERLIN AND R. T. CHAMBERLIN 



a moderate flood plain, giving the valley a measurably flat bottom. 

 It was observed that wherever the valley floor joins the steep valley 

 sides and might be expected to rise into them with a broad free curve, 

 there is instead almost universally a short sharp curve, almost an 

 angle. There is almost no wide open sweep of the surface from the 

 low slope of the bottoms to the steep slope of the valley sides, such 

 as is so common in the mountainous parts of the United States and 

 in higher latitudes generally, so far as we have seen them. In the 

 higher latitudes the foot of a steep slope of indurated rock of this 

 sort is usually buried beneath a belt of talus and mantle rock which 

 serves to merge the high declivity of the valley side by an open concave 

 swing into the low declivity of the valley bottom. In this tropical 

 Mexican valley the slopes are clean and steep all the way down to the 

 immediate vicinity of the flat bottom into which they turn almost at 

 an angle. On the slopes there is only a very scant sheet of mantle 

 stuff adhering to the rock in place, but yet this mantle supports a 

 luxuriant vegetation. The scree slopes, so familiar in like situations 

 in higher latitudes, are almost entirely absent. 



In January, 1909, during a brief visit at Honolulu, we were able 

 to make a hasty study of the Nuuanu Valley which leads up the 

 southern slope of the island of Oahu to the Pali, a striking viewpoint 

 on the backbone of the island. Compared with northern configura- 

 tions, this valley impressed us as unique in its profiles and, while 

 recalling the Mexican phenomena, was not altogether identical 

 with them. The mountain slopes on either side are exceptionally 

 steep, rising when at their maximum with angles of scarcely less 

 than 60°, as near as the eye could measure; and yet they are generally 

 clothed with vegetation, though the bare rock comes to the surface 

 at many points in the midst of the luxuriant vegetal growth. These 

 slopes are well creased by erosion trenches, giving the valley side a 

 corrugated aspect. Except locally, soil is not abundant in the upper 

 part of the valley, even on the bottoms. There appears to be little 

 or no talus at the base of the slopes, which curve sharply into the 

 valley bottom, much as in the Mexican case. The valley bottom 

 presents a general aspect of planeness, but is rough in detail and 

 usually quite rocky. 



One of the noteworthy features of the valley is its peculiar cross- 



