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University of California. 



[Vol. 



centage of soda, as shown by the bausch analysis. Opposed is the 

 low percentage of lime, and the possibility that the feldspar either 

 contains a sufficient amount of soda, or that it has been introduced 

 subsequently. In the light of all the facts it seems to the writer 

 that the hypothesis of the derivation of the analcite from nepheline 

 has the most in its favor, although several things are still unex- 

 plained. 



The rock as a whole has close affinities with those of the dis- 

 puted teschenite group, but contains no mica or hornblende. 

 Rosenbusch* holds that the analcite in rocks of this class, in which 

 nepheline cannot be detected, has nevertheless been derived from 

 nepheline, that if it were not derived from nepheline the rock 

 must have had originally a remarkable porous structure, and that 

 he does not see how the alteration of a lime-rich feldspar can result 

 in a soda zeolite. Such a porous structure (miarolitic) as must 

 have existed in the rocks under discussion, had the wedge-shaped 

 areas been left empty at the close of the solidification of the magma, 

 is unknown in rocks of the diabase type. 



Rohrbach,! who subsequently studied the rocks termed tesche- 

 nites by Tschermak, was unable to find nepheline in any of them, 

 and reached the conclusion that the analcite owed its origin to the 

 feldspar. His description of the manner of the occurrence of the 

 analcite is very similar to that observed in the Cuyamas eruptive. 

 He mentions analcite as present in angular spaces between the feld- 

 spars, also in shreds and veins with sharply defined boundaries 

 replacing them, pseudomorphs of analcite after feldspar also being 

 noted. The analyses given by him show, however, a very much 

 less amount of soda than is found in the rock under discussion. 



The feldspar in the rock studied by Rohrbach exhibited a zonal 

 structure, and he considered that it represented all degrees of basic- 

 ity from anorthite to albite. If this view is correct, the feldspar 

 would furnish quite a percentage of soda. Macpherson, in his study 

 of the teschenites of Spain, as cited by Rosenbusch, reached much 

 the same conclusion as Rohrbach. 



*Mikroskopische Physiographie der Massigen Gestine, p. 253. 

 fUeber die Eruptivgestine in gebite der Schlesich-Mahrischen Kreidefor- 

 mation. T. M. P. M., 1886. 



