PORPHTEITE. 335 



PRINCIPAL GROUP. 



The characteristic primary constituents of these rocks are the minerals zircon, 

 allanite, apatite, magnetite, biotite, hornblende, plagioclase, orthoclase, and quartz. 

 To these, as occasional accessories, may be added ilmenite and pyrite. All the common 

 non-essential elements are developed in the ordinary way, and none is so abundant 

 or so rare as to deserve comment. Allanite is not always present in the thin sections 

 examined, but its observed distribution among different types is such as to warrant 

 the belief that it is sporadically present in all the rock masses of this group. 



Feldspars All crystals of the first period of consolidation which have been 

 identified are plagioclase, with but one possible exception, referred to later (p. 339). 

 Orthoclase may be sparingly developed in this way in a few cases, but the freshness 

 of the plagioclase in nearly all specimens collected and the ease with which the stria- 

 tion can be seen upon the basal cleavage plane make it certain that a monoclinic 

 feldspar must be very rare. In the groundmass, on the other hand, plagioclase is not 

 visibly present at all in many cases, while orthoclase is very abundant. 



- The plagioclase crystals are small, white, stout in form, and correspond exactly 

 to those described in the quartz porphyries. They are chemically near oligoclase, 

 judging from the optical properties, for the maximum observed extinction in the zone 

 perpendicular to the usual twinning plane is but 20. In a number of crystals twin- 

 ning according to the Carlsbad law is apparently combined with that of the albite 

 law, as, for example, in one section, falling at right angles to the bracbypinacoid, there 

 are 20 laminae, of which five pairs extinguish sharply at 8 45', the other five pairs at 

 from the twinning plane. In a few cases more than two directions of extinction 

 were noticed in sections apparently lying in the macrodiagonal zone. In one crystal 

 laminae were found extinguishing at 1, 4 30', 8, 13, and 20, several pairs showing 

 the last two values. A satisfactory explanation for this action has not been found. 

 It may be that laminae of different feldspars are hereintergrown, but such a conclusion 

 must be supported by further data than are here available. 



A delicate zonal structure is occasionally seen in plagioclase crystals, but the 

 slightly varying angles of extinction do not indicate any pronounced changes in 

 basicity of the different zones. 



Biotite Biotite appears as a constituent iti three distinct forms: as macroscopic 

 hexagonal leaves, in aggregates of small irregular flakes, and as minute leaflets in the 

 groundmass. The large leaves are brown when fresh and often exhibit ragged edges 

 when seen tinder the microscope, caused by the attachment of many flakes correspond- 

 ing to those in the groundmass. Allanite, zircon, magnetite, and apatite penetrate 

 the larger leaves. The tiny leaflets which enter at times richly into the composi- 

 tion of the groundmass are irregular in shape and rarely over 0.03 mm in diameter, 

 sometimes sinking to a minuteness requiring the highest power of the microscope to 

 resolve them into separate flakes. They are greenish in color and at first glance it is 

 not easy to discover t'heir nature as mica; but their marked pleochroisin and strong 

 absorption in proper position renders their character certain. These flakes of green 

 mica are often arranged one after another, partially overlapping, making needle-like 

 aggregations, easily mistaken for hornblende with a low magnifying power. 



Hornblende The hornblende is compact, of a green color in ordinary light, and 

 generally presents quite well-defined crystals, the faces ccP, ccP do, OP, and P being 



