33 



University of California. 



[Vol. bL 



derivation from some primary mineral trace it to a feldspar, which, 

 according to all observers, has an average composition as basic at 

 least as labradorite. Zirkel * says, "It is not to be denied that it is 

 difficult to conceive of the alteration of plagioclase which always 

 contains more lime than soda into a soda rich analcite almost free of 

 lime." Rosenbusch f remarks to the same effect, and adds farther: 

 "It remains difficult of explanation how the zeolitizing of a lime 

 rich feldspar results almost exclusively in a soda rich zeolite. . . , 

 The relative quantities of the alkalies and silica make it probable 

 that nepheline was the original constituent." 



It seems to the author that when there are taken into account all 

 the facts connected with the occurrence of this teschenite-like type 

 at Point Sal, as well as at other localities in the Coast Ranges, it 

 can not be believed that the peculiarities are due merely to the infil- 

 tration of the elements forming the analcite after solidification. It 

 has been shown certainly that it can not possibly have been derived 

 from the feldspars, and to suppose that a miarolitic structure once 

 existed in all phases of the rock under such exceedingly varied con- 

 ditions demands a stretch of the imagination beyond all scientific 

 bounds. 



Although this rock differs considerably from most of the typical 

 teschenites of Europe, yet there are many common features, both 

 chemically and mineralogically, and it has seemed best to use the 

 name already introduced. The only explanation' which it appears 

 can account for this peculiar and uniform type of eruptive rocks as 

 known in California, is that there was originally present a soda rich 

 mineral, in all probability nepheline. 



BASALTIC INTRUSION IN THE KNOXVILLE. 



Occurrence. — A small body of dark basaltic rock appears in the 

 Knoxville nearly in the center of the larger area on the head of Cor- 

 ralitos Creek. The relation of this eruptive boss to the Knoxville 

 is quite remarkable and interesting. The soft shales have been 

 locally tilted up and hardened, and it is due to their more resistant 

 character that the underlying eruptive was discovered. It has 



*Lehrbuch der Petrographie, Vol. II, p. 681. 



f Mikroskopische Physiographic, 2d edition, p. 252. 



