272 Wolff — Occurrence of Theralite in Central America. 



rite of the composition AfrAn 1 . A few sections of untwinned 

 feldspar have an index of refraction inferior to that of the 

 balsam, a small axial angle, and are therefore probably ortko- 

 clase. 



The mineral of the sodalite group occurs exactly as in the 

 Montana rock in little crystals enclosed in the feldspar, with a 

 dirty blue color, isotropic character and characteristic square or 

 hexagonal outlines. The nepheline is in small stout square 

 prisms, generally in clusters or enclosed in the feldspar — the 

 usual fibrous decomposition has obscured the original optical 

 properties, but its entire identity in form and characters with 

 the nepheline of the weathered specimens of the Montana 

 rock, leaves no doubt as to its identification. Zeolitization has 

 spread from the nepheline to a large part of the feldspar ; most 

 of the zeolites are of the radiating prismatic type, which was 

 not further determined, but there are some patches of analcite. 



The main differences from the Montana type are in the 

 absence of segirine, the distinctly basic soda-lime feldspar and 

 the small amount of sanidine; it probably represents a type 

 poorer in alkalies, but a chemical analysis was considered unre- 

 munerative on account of the small amount of material and 

 considerable alteration of the rock. It represents the purest 

 type yet described of theralite as defined by Rosenbusch, 

 namely a soda-lime feldspar-nepheline, plutonic rock. 



From the point of view of there lationship of rock magmas 

 this occurrence is extremely interesting, representing as it does 

 a peculiar and rare rock type occurring some three thousand 

 miles from the Montana region, but in a somewhat analogous 

 position both as regards Tertiary age, and location some dis- 

 tance east of the main continental divide. It may be that 

 other occurrences will be found in the future in the interven- 

 ing region ; the nearest representative to theralite known to 

 me in the intermediate distance is the nepheline rock described 

 by Zirkel (in the " Microscopical Petrography of the 40th 

 Parallel") from Fortification Rampart, Elkhead Mts., Colorado, 

 of which more will be said in the forthcoming monograph on 

 the Crazy Mts. 



The lack of a chemical analysis makes a comparison from 

 the point of view of identity or difference of rock magmas of 

 less value and weakens the argument which could be based on 

 this occurrence against the localization of rock magmas, while 

 on the other band it points to a possible southern continuation 

 of the zone of alkaline rocks occurring in the northwestern 

 United States in the east border of the Rocky Mountains. 



University Museum, Cambridge, Mass., February, 1896. 



