C. W. Knight — Occurrence of Pseudo-Leucile. 289 



sections give a distinct uniaxial cross, negative in character. 

 The mineral does not seem to be abundant, but a more care- 

 ful examination with high powers shows it to be present in 

 greater amount than would at first appear. Wavy extinction, 

 as observed in the orthoclase, is likewise seen in the nephelite. 

 The biotite occurs in grains always less than - 05 mm in diameter. 

 It is a greenish brown variety, strongly pleochroic. The tiny- 

 plates tend to arrange themselves along lines ; again they are 

 segregated into patches l mm or so in diameter, or are irregu- 

 larly distributed throughout the pseudo-crystals. A very few 

 individuals of an acid plagioclase feldspar were noted. 



It has been already stated that the pseudo-leucites in speci- 

 men B do not appear to possess a crystal form when examined 

 macroscopically. Under the microscope, however, some crys- 

 tal faces are observed. It is possible that the absence of dis- 

 tinct crystal form may be due to the action of the magma on 

 the original leucite crystals. Apart from the form of the crys- 

 tals the microscopic description given above applies equally 

 well to the pseudo-leucites of both specimens. 



Nephelite: — A few crystals with quadratic cross-section and 

 low interference colors give uniaxial interference figures nega- 

 tive in character. They occur in the ground-mass with one 

 exception ; in this instance a crystal l«2 mm in diameter plays 

 the part of a phenocryst ; the mineral gelatinized easily in HC1. 

 Nephelite is not an abundant mineral in the rock. 



Scapolite : — Besides occurring as a constituent mineral of 

 the pseudo-leucite, scapolite also forms part of the ground- 

 mass. It is readily recognized by those characters already 

 enumerated above. In size the grains will average less than 

 •05 mm in diameter. 



Biotite : — The orthoclase laths and biotite make up the 

 principal minerals in the ground-mass. The biotite occurs in 

 plates, generally less than •l mm in diameter, lying in the inter- 

 stices of the feldspar laths. It is a greenish brown variety, 

 strongly pleochroic and appears to be similar to that found in 

 the pseudo-leucite crystals. 



Though nephelite does not play a prominent part, the rock 

 is nevertheless here classified as a pseudo-leucite phonolite. It 

 is highly probable, however, that nephelite was once present 

 in greater amount than it is now, since the scapolite appears to 

 be secondary after it. 



The pseudo-leucites studied in this paper resemble very 

 closely those from Brazil ; from Magnet Cove, Arkansas ; from 

 the Bearpaw Mountains, Montana; from Beemerville, JSL J.; 

 and from the Highwood Mts., Montana. O. A. Derby* de- 

 scribed the porphyritic types of foyaite from Brazil containing 



* Quart. Jour. Geol. Society, May, 1891, p. 251. 



