

F. L. Ransome — Lava Flows of California. 355 



Akt. XLV. — Some Lava Flows of the Western Slope of the 

 Sierra Nevada, California ;* by F. Leslie Eansome. 



Introduction. — The field relationships of the rocks described 

 in this paper were studied during the summers of 1895 and 

 1896, while engaged in geologically mapping portions of the 

 Sonora and Big Trees quadrangles, under the direction of Mr. 

 H. W. Turner. It was found that in addition to the andesitic 

 tuffs and breccias which are so widely and monotonously devel- 

 oped over the broad western slope of the Sierra Nevada, there 

 occurs within the district studied another series of lavas in the 

 form of massive flows, covering much less extensive areas than 

 the clastic andesites. Taken as a whole they form a set of 

 superimposed flows, extending in a southwest direction from 

 some as yet unknown source near the crest of the range, down 

 to the village of Knight's Ferry, near the edge of the Great 

 Valley — a distance of more than 60 miles. Only one of the 

 flows is known to have extended the whole distance, and they 

 have all suffered considerable dissection through the very 

 incisive gashing of the present streams ; but in a general way 

 the line of flow may be said to follow the present course of the 

 Stanislaus River. These lava-streams were poured out within 

 the period of eruptive activity during which the andesitic 

 breccias and tuffs were spread for hundreds of square miles 

 over the western flanks of the Sierra. But their eruption was 

 immediately preceded by an interval of sharp erosion, which 

 cut out the channel down which they flowed toward the valley. 

 Subsequently they were again partly covered by fresh deposits 

 of andesitic breccia, especially in the higher portions of the 

 range. 



The distinctive chemical feature of these rocks is a rather 

 high percentage of total alkalies, with the potash somewhat in 

 excess of the soda. Chemically they stand between typical 

 andesites and typical trachytes, and belong to a group which it 

 seems necessary to classify under a new name. Without 

 anticipation of a discussion which has its proper place after the 

 rocks themselves have been described, the name latite (derived 

 from the Italian province of Latin m or Latia, where rocks 

 closely related to those described in the present paper occur 

 extensively) may be considered, for the present, merely as a 

 convenient designation for the group of lavas which form the 

 subject of the present paper, for the purpose of avoiding con- 



* Published by permission of the Director of the U. S. Geological Survey; 

 being an abstract of Bulletin No. 89 of the Survey. 



A.M. Jour. Sci.— Fourth Series, Vol. Y, No. 29.— Mat, 1898. 

 24. 



