COMPONENTS OF THE SYENITE. 247 



idiomorphic grains, having traces of having undergone resorption, wo 

 must conclude that, like the augite and the biotite, it is primary in origin 

 and not secondary, as is the bright green hornblende. 



The colorless components forming the mass in which the dark aggre- 

 gates lie are sodalite, eleolite and feldspar, whose relative ages are proba- 

 bly in the order named. The first two mentioned are in small quantity 

 as compared with the feldspar, though the eleolite is in sutficient abun- 

 dance to characterize the rock as an eleolite-syenite. When unaltered it 

 is perfectly colorless. It occurs occasionally in prismatic * forms between 

 the feldspar, but more frequently as irregular masses associated with the 

 basic constituents of the rock and often surrounding them, and also as 

 grains included in the intergrowths of albite and orthoclase. The time 

 of its formation consequently was between that of the bisilicates and that 

 he feldspar. The inclusions in the eleolite, besides the sphene and 



dicates already mentioned, are flakes of a brightly polarizing, fibrous 

 abstance, and tiny grains of calcite. Both of these are decomposition 

 products of their host, for as they increase in quantity the eleolite sur- 

 rounding them gradually loses its transparency and other characteristics 

 until finally it passes into a cloudy mass, consisting largely of a felt of 

 the brightly polarizing fibers, studded here and there with grains of calcite. 



The sodalite is distinguishable from the fresh eleolite only in polarized 

 light, where it remains dark during an entire revolution. It occurs under 

 conditions that are exactly' similar to those under which eleolite exists. 

 It is found cementing the bisilicates in the basic aggregates, and is often 

 present as inclusions in the feldspar. Rarely is it discovered in pieces 

 of any size between grains of feldspar. Perhaps its most characteristic 

 form of occurrence is as inclusions in the feldspar. These are usually 

 very irregular in shape, but occasionally the grains show very clearly 

 the traces of dodecahedral planes (figure 2). That the isotropic grains 

 are sodalite and not some other regularly crystallizing mineral maybe 

 beautifully shown by Lemberg's test,t in which a dilute acid solution of 

 silver nitrate is allowed to come in contact with the uncovered section. 

 In a portion of a slide treated in this way the isotropic grains were covered 

 with :i white coating of silver chloride, while the nepheline grains re- 

 mained unaffected. 



The sodalite, like the eleolite. is older than the feldspars, hut is younger 

 than the bisilicates. A single observation upon the relative ages of the 

 first two mentioned numerals indicate3 thai the sodalite preceded the 

 eleolite in the time of its format ion. 



*It i- probably this that was taken by Hawes for quartz (see description, p. 243). 

 t J. Lembergi Zeits. d. d. geol. Gesell., b. \lii. L800, p. 738. 



XXX III r.i ii, Geol, Soi . \ n., \ ol. 3. L891. 



