GEOLOGY OF CARRIZO MOUNTAIN, CALIFORNIA 341 



H. G. Hanks, "Mud Volcanoes and the Colorado Desert," 2d 

 Ann. Report, State Mineralogist of California, 1880-82, 227-40. 



Chas. R. Orcutt, "The Colorado Desert," loth Ann. Report, 

 State Mineralogist of California, 1890, 899-919. 



Dr. Robt. E. C. Stearns, "The Fossil Freshwater Shells of the 

 Colorado Desert, Their Distribution, Environment and Variation," 

 Proc. U.S. Nat. Museum, XXIV (1902), 271-300. 



GEOLOGY 

 GENERAL 



Carrizo and Black mountains are islands of granitic and meta- 

 morphic rocks, which rise through encircling terranes of later sedi- 

 ments and volcanics. These later beds are Miocene and younger, 

 and the unconformity which exists between them and the older rocks 

 upon which they lie is profound. The time interval represented by 

 this unconformity is not known because the age of the altered rocks 

 below it is a matter of uncertainty. Fairbanks^ expresses the opinion 

 that they are Carboniferous or older, the opinion being based pre- 

 sumably upon their general resemblance to upper Paleozoic rocks in 

 other parts of California and upon the aspect of some shells found in 

 a float piece of siliceous limestone. Accepting this determination as 

 the best possible in the present state of- our knowledge, wt must con- 

 clude that the Triassic, Jurassic, and Cretaceous systems are without 

 depositional representatives in this region. Either the Carrizo and 

 Black mountain areas were land masses subject to erosion during 

 this interval or the evidence of such periods of deposition as intervened 

 was later removed by erosional processes. 



The Miocene seems to have been inaugurated by volcanic activity. 

 On the southern slopes of both Carrizo and Black mountains are 

 bedded tuffs, volcanic conglomerates, and less extensive masses of 

 dark lavas of andesitic aspect. On Black Mountain there are dis- 

 tinct sandstones interbedded with these and directly upon them lie 

 the Miocene coral reefs. In Alverson Canyon, which drains south 

 from Carrizo Mountain, red vesicular lavas are succeeded by green 

 and lavender sandstones and conglomerates, whose constituent 

 materials are volcanic, and these in turn grade into conglomerates 



I Fairbanks, H. W., nth Kept., California State Mineralogist, 1893, 88, 90. 



