298 



University of California. 



[Vol.i. 



to the so-called teschenites have been found in the United States. 

 While the original presence of nepheline has not been positively 

 demonstrated, the phenomena shown mostly favor that supposition. 

 The designation analcite diabase is used, although it seems to the 

 writer that the rock really belongs among the theralites. 



The most important peculiarities of the rock are the following: — 



The combination of diabase granular or ophitic structure with 

 the panidiomorphic, the augite in the main body of the Cuyamas 

 dike being allotriomorphic, in the smaller secondary dikes partly 

 or wholly idiomorphic. 



High extinction angles in the feldspar associated with a low 

 percentage of lime, as shown by the analysis. 



The existence in the augite of a peculiar parting, which, while 

 conforming to the position of the diallagic cleavage, yet seems dis- 

 tinct from it. 



The beginning of alteration in the central portion of the augite 

 crystals, resulting in silvery white, diallagic cleavage faces. 



The idiomorphic forms of both feldspar and augite with ref- 

 erence to the analcite areas. 



The presence of a large percentage of analcite under four con- 

 ditions, namely: Crystallized and lining cavities in the rock; filling 

 angular spaces between the older components; replacing the feld- 

 spars; and in the form of hexagons, idiomorphic with reference to 

 the feldspar. 



Prehnite, supposed to have been formed either in the analcite 

 or during its derivation from some primary component. 

 Secondary feldspar replacing the analcite. 



High percentage of soda and absence of sulphur and chlorine, 

 pointing strongly to the presence primarily of nepheline. 



Presence of secondary spherulites of an indeterminate mineral in 

 one facies of the rock. 



Geological Laboratory, 



University of California, January 75, 1895. 



