GREAT DEGRADATION OF THE DISTRICT. 95 



are complete and symmetric ; few of the outlying sierras have escaped 

 trenching by the retrogression of arroyas opening westward or southwest- 

 ward, and many of the divides, main as well as minor, have been shifted 

 from the rims well into the interiors of the valley-plains. 



Geologic development. — The observations of the two expeditions by the 

 Bureau of American Ethnology yielded certain inferences concerning 

 the structure, geomorphy, and geologic history of the district, and these 

 in turn yielded certain generalizations; and as a supplement to the 

 empiric description they are worthy of statement for the purpose of 

 making clearer the features of the district, though it is to be re- 

 membered that they constitute a series of field hypotheses, probably 

 valid in the main though requiring elaboration and minor modifica- 

 tion with further field-work. The first inference is that the clastic rocks 

 represent a thick and practically continuous series of deposits laid 

 down on a granitic floor, the series growing finer and more calcareous 

 southward. The second inference is that about the close of the period 

 of deposition vulcanism was initiated, whereby considerable sheets 

 of lava and tuff were produced. The third inference is that toward 

 the close of the period of vulcanism extensive deformation occurred, 

 chiefly as east-west compression, with consequent development of merid- 

 ional corrugations, and it is deemed probable (though there is little 

 explicit evidence on this point) that the cycles of (1) deposition, (2) vul- 

 canism, and (3) deformation occurred in this order, yet overlapped to 

 such an extent that deformation perhaps began before deposition entirely 

 ceased and while sporadic vulcanism also persisted. The fourth infer- 

 ence is that the massif produced in this way stood at moderate altitude 

 for a long period, including approximately the Eocene and the earlier 

 half of the Neocene ; that a large part of its volume was degraded ; that the 

 surface was planed to an approximate baselevel, relieved by ridges and 

 masses of the monadnock and catoctin types, usually of harder layers 

 but sometimes marking broader divides, and that during this vast period 

 the drainage basins were outlined and developed. It is deemed prob- 

 able that during much or all of this period the precipitation was greater 

 than now, so that the district throughout was one of degradation, and 

 so that the drainage basins were of the normal dendritic type, veined by 

 rivers occupying broad yet essentially V-shape valleys ; and it is consid- 

 ered probable also that the basin-limiting sierras were less rugged than 

 now. The final inference is that the period of symmetric degradation 

 was terminated by general south westward tilting due to uplift in the 

 northeastern portion — the uplift that resulted in the making of the Sierra 

 Mad re in Mexico and the high plateaus in the United States, and in the 

 birth of Colorado river ; that this geographic change was accompanied 



XIV— Bull. Geol. Soc. Am., Vol. 8, 1896 



