GEOLOGY OF THE THREE SISTERS, OREGON 499 



This group of peaks is parked by the presence of a glacier nearly three miles 

 long and a half a mile wide. 



To the north of the peaks recent volcanic activity is indicated by extensive flows 

 of basic lavas. Volcanic eruptions have occurred since the Glacial period, as 

 shown by the relation of the lavas to the grooved and polished surfaces. A volcanic 

 cone on the North Sister lies in the path of the present glacier. 



Petrographically the region is also an interesting one. 



At 12.30 p m the Section adjourned for luncheon. 



After luncheon the following papers were read and discussed : 



SKETCH OF THE PEDOLOGICAL GEOLOGY OF CALIFORNIA 

 BY E. W. HILGARD 



[^Ahsiract] 



Owing to the great climatic diversity, the rainfall varying from 2 inches at the 

 south to as much as 80 inches in the north, even a sketch of the soil conditions of 

 California must take the climates into consideration. The cardinal difference 

 between rock decomposition in arid as compared with humid climates lies in the 

 retardation of kaolinization, as exemplified in the monoliths of Egypt and the 

 granites of the Sierra Madre, as compared, for example, with the Alleghanies. 

 Hence in northern California and on the higher Sierra Nevada we find loams and 

 clay soils, while at the lower levels and in southern California the soils are " dusty " 

 or sandy, except where derived from preexisting clay formations, which give rise 

 to '* adobe," and in the upper valleys of the rivers of the Sierra, which carry the 

 materials from the higher levels. 



Throughout the middle and southern parts of the State, where no rains of con- 

 sequence fall between May and November, not only is the soil mass usually of 

 extraordinary depth, but is scarcely changed for several, sometimes for 4 to 10, 

 feet. There is practically no subsoil, in the usual sense, in the absence of clay ; 

 water, roots, and air penetrate together to depths impossible in the regions of 

 summer rains, and hence the extraordinary endurance of drought, even by plants 

 foreign to the arid region. Moreover, these soils almost universally contain high 

 percentages of lime and potash, due to the absence of the leaching process, which, 

 on the other hand, results in the formation of " alkali soils " — too complex a sub- 

 ject to be dealt with here. 



"Sand" in the arid soils is not merely quartz grains, bat consists of all the 

 original minerals, superficially decomposed. Hence sandy lands are here fully as 

 rich as clay lands are elsewhere. 



In the Great valley it is easy to recognize by their microscopic characters the 

 alluvial areas of the several rivers coining in from the Sierra. Even here the 

 greater rainfall of the Sacramento is evidenced by loam and clay lands, as com- 

 pared with the San Joaquin valley, where sandy and silty lands prevail altogether. 

 As the rainfall decreases toward the Coast ranges, the " lightest" are found under 

 the arid lee on the w^est border of the valley, in its axis, in the " tule lands," as 

 well as on the borders of the bays near its outlet, heavy clay soils are being formed 

 in the slack water, while the streams coming from the Coast ranges are bordered 



