6 PKOCEEDINGS OF THE NEW YORlv MEETING 



various plagioclase. Cleavage is good after both (001) and (010). The rock given 

 under analysis I was pulverized by stamping and the minerals isolated by means 

 of the Klein solution. A small portionof feldspar had the specific gravity 2.69-2.()84 

 at 22 degrees centigrade. Extinction angles on P (001) were found to range about 

 8-10 degrees, while on untwinned pieces, evidently after M (010), values of 20-23 

 degrees were found. Most of the feldspar has a specific gravity of 2.684-2.64 at 

 22 degrees centigrade, in which angles on P (001) were found from 0-7 degrees, 

 while a few angles of 12-17 degrees were observed on particles evidently cleaved 

 after M (010). The latter gave in convergent light an axial figure intermediate 

 between that of oligoclase and labradorite. Another considerable portion ranges 

 from 2.64-2.604 at 22 degrees centigrade, but it was found to consist mainly of 

 groundmass; in fact, the portion from 2.684-2.64 had considerable groundmass 

 mixed with it. 



The foregoing observations, together with the maximum symmetrical extinction 

 angles given in sections normal to M (010), indicate that the ])henocrysts are mem- 

 bers of the andesine and acid labradorite series. The successive occurrence of 

 these,* with even other plagioclase not mentioned, is not to be wondered at in 

 view of the frequent zonal structure ; since particles belonging to the same crystal 

 may give extinction angles ranging from those of labradorite to oligoclase, as was 

 observed in a few cases. It is extremely probable that the plagioclase phenocrysts 

 of the majority of the rocks are more basic than in the present instance, which is 

 the most acid rock of the series. Symmetrical extinction angles of 25-27 degrees 

 in the zone normal to (010) with Albite twins and Karlsbad twins with differences 

 in the two sides of 12-15 degrees would seem to point out that the more basic lab- 

 radorites play an important part in many of the rocks under investigation. The 

 plagioclase decompose first along cleavage and twinning directions, except perhaps 

 when zonal structure is present, in which case the alteration begins at the center 

 of the crystal. The secondar}' minerals resulting from the feldsiiars were found 

 to be calcite, mica, kaolin, ej)idote, and possibly chlorite, as the result of nuitual 

 reaction of solutions originating from plagioclase and the dark minerals. Quartz 

 was at times present in small amounts, but whether it resulted from the feldspars 

 or other minerals could not be stated. Of these alteration products calcite was 

 probably most common, then come mica, either paragonite or muscovite, and, 

 finally, epidote, the remainder of the decomposition products being rarer. 



Hornblende is the next most important constituent after the plagioclase. It is 

 pleochroic with c and h = brown, <l = yellow with a tinge of brown. The crys- 

 tals are generally idiomorphic, being bounded by the prism (110), clinopinacoid 

 (010), and terminated evidently ])y pyramid and dome faces. At times, however, 

 the mineral occurs in bizarre forms, which are due to resorption. Twins were ob- 

 served after the orthopinacoid (100). Cleavage is good after the prism (110) and 

 cleavage fragments gave an extinction of 11-13 degrees, which would probabl)' 

 yield values of 14-16 degrees on (010). Indications of zonal structure were also 

 observed. All of these facts indicate a typical basaltic hornblende. Through de- 

 composition the mineral loses its color and the pleochroism becomes weaker, but 

 the double refraction does not change. Along the outer parts of the mineral a 

 mass of fine dark grains of iron oxide, which at times is hematite, accumulates, 

 giving the hornblende at first glance an appearance of zonar structure, or perhaps 

 of resorption rims. This could not be the fact, however, as the rims increase in 

 width with growing decomposition. In some cases the alteration does not pro- 



