7 8 EDITORIAL 



several instances known to have been evolved after the deformation of 

 the Pliocene, and we are thus forced to place a very late geological 

 date upon the tilting of the Sierra Nevada orographic block. 



A Feldspar- Corundum Rock from Plumas County, California. By 

 Andrew C. Clawson, Berkeley, Cal. 



Mr. Turner, of the United States geological survey, has called 

 attention to the prevalence of feldspathic "albitic" dykes cutting 

 serpentine in various parts of the Sierra Nevada. The rock of which 

 the present paper treats apparently belongs to this series of dykes. It 

 occurs as a white coarse-grained dyke cutting the serpentine of the 

 eastern flank of Spanish Peak, Plumas county. The rock is composed 

 of 84 per cent, of oligoclase and 16 per cent, of corundum in crystals 

 up to over two inches in length, and rather irregularly distributed 

 through the feldspathic groundmass. 



The following is an analysis of the feldspar : 



Si0 2 , 61.36; A1 2 3 , 22.97; Na 2 0, 8.08; CaO, 5.38; H 2 0, 1.72. 

 Total, 99.51; Sp. g, 2.63. 



The occurrence is of special interest as one of the rare cases of a 

 rock supersaturated with alumina, and its occurrence as a dyke in a 

 rock devoid of alumina, soda, and lime is of especial interest as 

 supporting a case of extreme differentiation of rock magma. 



The foregoing synopses were prepared by the authors of the 

 papers. 



John C. Merriam. 



