746 



SCIENCE. 



[N. S. Vol. XV. No. 384. 



and to John Playfair we owe the elucidation 

 of these ideas, and their amplification. 



The doctrine that rivers are the cause of 

 their valleys, and the proof thereof is perhaps 

 the most important foundational idea that 

 we owe to the combined labor of these two 

 geological worthies. Playfair's clear exposi- 

 tion of the possible origin of river terraces, 

 his acute description of the relation of lakes 

 to rivers, his analysis of the varied forms of 

 shore lines, and his emphasis of the impor- 

 tance of initial shore lines, all clearly ex- 

 ploited in his illustrations, deserve to take 

 rank with the much-quoted passage on rivers 

 and their valleys, as being accepted geograph- 

 ical truths far in advance of their time. 



After the reading of these memorials the 

 Section listened to two papers by Professor R. 

 E. Dodge and one by Gilbert van Ingen, all 

 of which were illustrated by means of the lan- 

 tern. 



Professor Dodge's first paper was entitled 

 'An Interesting Landslide in the Chaco 

 Canon, New Mexico,' and he said in brief: 



On a high mesa to the southeast 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 rim rocks of an old escarpment 

 three hundred feet high. The rim rock of the 

 escarpment is a coarse brown sandstone 

 capped by about two feet of thin-bedded 

 dark brown sandstone containing sharks' 

 teeth. The face of the escarpment has 

 recently slipped along a series of joints 

 running approximately parallel to face of 

 escarpment, and in a general direction of 

 S. 30° E. The recesses between slipped 

 blocks can be sounded to .a depth of over fifty 

 feet, and are wider at base than at top as a 

 rule. 



In the slipping an ancient rock hogan 

 twenty feet in diameter has slid 2.5 feet verti- 

 cally and 8.3 feet horizontally without displac- 

 ing 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 ob- 

 servation for several years, seems to show 

 that the work has changed from aggradation 

 to degradation because of some influence that 

 has caused the focusing of the running 

 water. Such a concentration of water is 

 made possible by over-grazing of the land, 

 which removes the help of 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 ero- 

 sion is difiicult, because of the tendency of 

 arroyos cut in adobe to maintain nearly verti- 

 cal walls, and because a fallen block of adobe 

 may be sealed over in the next fiood, so that it 

 looks in place. This problem is of especial 

 importance, because the adobe deposits in 

 some places contain relics of human occupa- 

 tion to a depth of many feet. The exact or 

 even the approximate antiquity of the de- 

 posits cannot be definitely determined, be- 

 cause of the several ways in which the order 

 of events in such a case may be interpreted. 



Mr. van Ingen's paper was on 'The Ausable 

 Chasm,' and gave a description of the geology 

 and physical features of this celebrated 

 locality which incorporated the results of the 

 author's own observations with those which 

 had been arrived at and published by others. 

 Edmund O. Hovey, 

 Secretary. 



BIOLOGICAL SOCIETY OP WASHINGTON. 



The 354th meeting was held on Saturday 

 evening, April 19. 



Barton W. Evermann and E. L. Golds- 

 borough presented 'Notes on Some Mexican 

 Fishes,' based upon collections made in 

 Mexico and Central America by Mr. E. W. 

 Nelson, Dr. J. N. Rose and others. Attention 

 was called to the occurrence of a species of 

 Cichlid (Heros uropMlialmus) in the cenotes 

 or natural wells of Yucatan. These wells 

 occur in a region where there is no surface 

 water, and it is diificult to account for the 

 presence of fish in them. 



Mr. Nelson found this same species in salt 

 water at Progreso and Mujeres Island, on the 



