and its Contact Phenomena. 28 
appearance due to the development of Carlsbad twins of ortho- 
clase with rounded contours, in a gray fine-granular aggregate 
of granitic minerals, which are said to form a mixture resemb- 
or in 
teristic. Its twin crystals of feldspar in polarized light are 
seen to have the peculiar structure of perthite, and consist of 
interlaminated orthoclase and albite.* Individual grains of a 
green, yellow, dichroic, in thin sections, and peculiarly impure 
from the enclosure of quartz grains. Biotite, magnetite and 
apatite are constant, augite and fluor spar are frequent, con- 
stituents, 
_ Bat what gives to this rock a very marked microscopic 
individuality is the uniform presence in it of well-crystallized 
Square prisms of zircon. Of the many sections that have been 
cut, not one has been found free from these pretty crystals. 
They are large enough to be examined optically under the 
microscope, and are easily recognized by their tetragonal 
crystallization, their high index of refraction. Their uniaxial 
and positive character can be easily determined in convergent 
ig! t. Out of twenty-five grams of the roc om Mt. 
illard, I separated several hundred of these crystals, by 
means of hydrofluoric acid. They are white, clear and 
glassy, but are sometimes tinged with yellow. They are often 
_mm. in diameter and mm. long. Their surfaces are 
bright, but cavities often penetrate far into their interiors. 
They are doubly terminated, and in addition to the planes of 
the prism and pyramid of the first order, they frequently have 
the planes of a ditetragonal pyramid which is probably the 
form 3-8. They contain many inclusions. Some of these are 
the inverted forms of zircon crystals, some are zircons with 
different terminal faces, and some are empty cavities with very 
regular forms. 
n the middle of the arm of Albany granite which extends 
across the summit of Mt. Willard, the rock is of this normal 
character, but both to the right and the left differences are evi- 
*g Sections parallel to the base hardly show these interlaminations owing to the 
a in the elasticity planes of the two cies. Sections parallel to the 
a - Pinacoid possess an elasticity plane making an angle of” ° with the asal 
th. ge, and in the interlaminations an elasticity plane makes an angle of 17° in 
© same direction, with the basal cleavage. 
