University of California. 



[Vol. i. 



perfectly clear, does not polarize brightly, and is unaffected by acids. 

 The large areas show the dark arm of a biaxial interference figure. 

 The manner of occurrence would lead to the belief that it is a soda- 

 rich feldspar. Several instances were observed of a twinned feld- 

 spar, fresh in appearance, and showing a small extinction angle, 

 which was also inclosed in the analcite and apparently of secondary 

 origin. One of these lias a regular and well-defined crystal bound- 

 ary. Secondary feldspar is also found in and on the edges of the 

 primal')' feldspar. 



Titaniferons Magnetite. — The oldest separation from the magma 

 is magnetic iron oxide containing titanium. It occurs in irregular 

 grains, hexagonal and rectangular forms, and long slender rods 

 sometimes united in skeleton crystals. These penetrate all the 

 other primary components of the rock. In some of the hand speci- 

 mens these rods are plainly visible, being a millimeter or less in 

 width and ten to fifteen millimeters long. It is present in all the 

 dikes, but the proportion varies greatly. Plate 16, Fig. 4, illustrates 

 a rock in which there is an uncommonly high percentage of this 

 mineral. 



CHEMICAL CHARACTER. 



The writer is indebted to Mr. V. Lenher, of the Department of 

 Chemistry of the University of California, for the analysis (I) of the 

 large secondary dike in the Cuyamas region given below. There is 

 inserted for comparison, an analysis (II) given by Rohrbach* of a 

 typical teschenite; of a theralite (III) by Wolff |; and of a plagio- 

 clastic eleolite syenite (IV) from Arkansas.^ 



*T. M. P. M. 1886. 



tNotes on the Petrography of the Crazy Mountains, Montana. 

 JGeological Survey of Arkansas. Igneous Rocks, p. 139. 



