248 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the pyroxene core is replaced by many subradiate to diverse, aegirite 

 prisinoids. The optical properties of the augite are apparently normal, 

 but with the de^'elopment of the green material these properties be- 

 come rather indefinite, the double-refraction is too low and the 

 extinction too large for aegirite. The most characteristic thing about 

 this pyroxene material is the rich yellowish-green to deep-green shade 

 belonging to the alkali-pyroxenes. Scattered throvigh the groundmass 

 are fairly good sized crystals of rich green pyroxene which show no 

 trace of augite and would be taken for aegirite or aegirite-augite, and 

 yet, their double-refraction is also low except about their margins, and 

 the optical properties are poorly defined. The characters just out- 

 lined may indicate that augite in small quantity was crystallized at an 

 early period (enclosure in the feldspars) and that subsequently as the 

 concentration of the aegirite molecule increased, the augite was not 

 only encrusted by the aegirite but became in large measure replaced 

 by it. It should be noted that the feldspars in which the augite 

 occurs resemble strongly the feldspars to be described as occurring in 

 the more basic phases, which are marginal differentiates of the magma, 

 and the suspicion is strong that the augite (and its enclosing feldspar) 

 represent crystals which have formed, early in the period of crystalliza- 

 tion. The analyses indicate that, as in the rhombenporphyry, the 

 augitic looking pyroxene is rich in the CaF"-molecule. That true 

 aegirine-augite is present the writer can find no \'ery positive proof. 

 The bulk of the aegirite is found as minute and usually irregular 

 grains scattered through the groiuidniass. 



Grains of a red to dark-red mineral, belie^■ed to be aenigmatite, 

 occur embedded in the green pyroxene and in the hornblende. The 

 same mineral has also a very characteristic and fairly abundant 

 distribution through the groundmass. Its mode of occurrence may 

 perhaps be described as "clustered." That is to say, it is made up of 

 from perhaps ten to one hundred or more minute grains, for the most 

 part unconnected, lying close together amidst the quartz, feldspar and 

 aegirite grains of the groundmass (see Figure I, Plate 1). The grains 

 are irregular in form with perhaps a slight tendency to an elongation 

 parallel to a poorly developed cleavage. The larger ones will hardly 

 exceed a few hundredths of a millimeter, the rest ranging down to 

 tiny round particles 0.005 mm. in diameter. In a single cluster the 

 majority of the grains have nearly or quite the same orientation. 

 Their color, and such other of their optical properties as can be made 

 out, seem to be the same as those given earlier for the aenigmatite 

 of the granite. The larger grains are commonly slightly attached to 



