20 BUREAU OF AMERICAN ETHNOLOGY [bull. 54 



creating a formidable obstacle to irrigation projects in the rapid 

 filling up of reservoirs. This condition does not permit of uniform 

 distribution of the soil from the mountain sides over the lower mesas 

 and bench lands, sucli as obviously took place in earlier times. A 

 humid clmiate, characterized by much rainfall in steady, long-con- 

 tinued showers, would cause a constant gradual degradation of higher 

 slopes and redistribution of the material upon the lower bench lands 

 in the uniform manner here seen. Furthermore, the Rio Grande and 

 many smaller streams show evidence of volume formerly much 

 greater than at present. 



The clmiate of the Southwest may have been at one tune similar to 

 that of Mexico in the high altitudes. No reason exists for believing 

 that any great physical catastrophe occurred to change conditions. 

 There is evidence of seismic activity but of not excessive violence. 

 This is seen in such shallow canyons as the Pajarito, where enormous 

 bowlders, fragments of the tufa escarpments, have been projected to 

 so great a distance from the cliff onto the level flood plains that it is 

 necessary to assume some initial force other than tlie mere breaking 

 away of the rock masses in the natural course of weathering. There 

 are evidences that the country has undergone a slow progressive 

 desiccation, extending over a long period of time. That the epoch 

 of human occupancy of this region extends back into the period of 

 greater humidity seems probable. Agriculture, without irrigation, has 

 flourished on considerable areas of the Pajarito plateau that are now 

 nonproductive from lack of water. That plateau anciently sup- 

 ported a large agricultural population where now it supports none. 

 On areas like Mesa del Pajarito, where the aggradation of a hundred 

 years under present climatic conditions would be almost imper- 

 ceptible, where in fact no distribution of soil from higher levels is 

 now in progress, there has been upbuilding of the general mesa level 

 about the walls of ancient buildings to an extent that is not explicable 

 under present conditions. The structures here referred to are not 

 the large community houses, but of an older scattered "small-house" 

 type, the predecessor of the many-chambered pueblo. 



While it thus seems probable that man witnessed these clhnatic 

 changes, the age to which we would have to return to find a condition 

 so different from the present can be fixed only within certain geologic 

 limits. The gravels above referred to are considered late Pleisto- 

 cene; that they are post-Tertiary is certain. In places they are 

 found overlying the lava flows, and no instances have come to the 

 writer's knowledge in which the reverse of this is the case. This 

 makes necessary consideration of the question of the age of the New 

 Mexico basalts. On this is here quoted Prof. R. T. Hill:^ 



In Bulktin of the Oeological Society of America, m, p. 100, 



