322 The American Geologist. May, i902: 
we owe to the combined labor of these two geological worthies. 
Playfair’s clear exposition of the possible origin of river terraces, his 
acute description of the relation of lakes to rivers, his analysis of the 
varied forms of shorelines, and his emphasis of the importance of 
initial shorelines, all clearly exploited in his “Illustrations,” deserve 
to take rank with the much quoted passage on rivers and their valleys, 
as being accepted geographical truths far in advance of their time. 
Professor Dodge also read a paper entitled: “An interesting Land- 
slide in the Chaco Canon, New Mexico.” On a high mesa to the south- 
east of the Chaco Canon, and about four miles below Putnam, New 
Mexico, is a series of stone monuments about five feet high and four 
feet in diameter, These monuments stand on the edge of a rim rock 
of an old escarpment nearly 300 feet high. The rim rock of the 
escarpment is a course brown sandstone capped by about two feet of 
thinbedded dark brown sandstone containing sharks’ teeth. The face 
of the escarpment has recently slipped along a series of joints running 
approximately parallel to the face of the escarpment, and in a general 
direction of S. 30 E. The recesses between slipped blocks can be 
sounded to a depth of over so feet, and are wider at the base than 
at the top as a rule. 
In the slipping an ancient rock hogan twenty feet in diameter has 
slid 2.5 feet vertically and 8.3 feet horizontally without displacing 
the rock walls to any serious extent. 
The second paper by the same author, was on “Arroyo Formation.” 
An arroyo is a steep sided, narrow gulch cut in a previously filled 
gravel and adobe valley in the arid west. 
The study of the process of formation of arroyos, some of which 
have been under observation for several years, seems to show that the 
work has changed from aggradation to degradation because of some 
influence that has caused the focussing of the running water. Such 
a concentration of water is made possible by overfeeding of the land, 
which removes the help af roots in holding soil particles, combined 
with the habit of cattle to move in processions along trails that make a 
natural channel -for water. 
The study of the rate of valley filling or erosion is difficult, because 
of the tendency of arroyos cut in adobe to maintain nearly vertical 
walls, and because a fallen block of adobe may be sealed over in the 
next flood, so that it looks in place. This problem is of special im- 
portance, because the adobe deposits in some places contain relics of 
human occupation to the depth of many feet. The exact or even the 
approximate antiquity of the deposits cannot be definitely determined 
because of the several ways in which the order of events in such a case 
may be interpreted. 
Mr. van Ingren’s paper was on “The Ausable Chasm” and detailed 
some of the results of the author’s own observations on that cele- 
brated locality besides referring to the work of others on the geology 
and physical features of the region. 
EpmMunp QO. Hovey Secretary. 
