326 Bulletin of Laboratories of Denison University [Voi. xi 
times in size. These larger crystals are mostly fresh and unal- 
tered, with high extinction angles, suggesting anorthite. The 
twinning phenomena are very well developed, according to both 
the Albite and Carlsbad laws. The mass of the rock, however, 
is made up of small, lath-shaped crystals, having the same opti- 
cal characters as those first described. These smaller crystals 
betray a'tendency to a fluidal arrangement, and the larger plagio- 
clases are sometimes broken by transverse fracture lines, appar- 
ently as the result of internal tension. Next in point of abun- 
ance are scattered irregular crystalline grains, often in polyso- 
matic groups, which are of an intense red color and permeated 
by irregular crevices. This mineral polarizes in shades of red 
and brown, behaving much like hematite. In a few cases there 
is a suggestion of the original olivine. The augite consists of 
minute scattered grains in the interstices of the second order of 
plagioclases. Grains of magnetite are freely scattered through- 
out the section. The cavities are frequently partially filled in 
with amorphous material. 
No. 504. Massive portion of the uppermost flow at White 
Rock Cafion. Hand-sample an intensely black, homogenous rock 
with a splintery fracture and occasional gas pores. To the un- 
aided eye, the ingredients appear quite unaltered. Portions of 
this flow are laminated and break into schistose fragments. Thin 
section densely sprinkled with minute black granules, giving to 
the section a peculiar opaque character. The largest ingredi- 
ents are well formed, tabular crystals appearing in section as 
elongated rectangles, having a rough surface, brilliant polariza- 
tion, and intersected by conchoidal crevices, tending to lie 
transversely to the longer axis. These crystals are often much 
broken and penetrated by the magma. The sections extinguish 
parallel to the longer axis and are little altered or decomposed, 
and constitute a remarkable occurrence of olivine. Occasional 
crystals of the same material have truncated planes. Plagioclase 
is abundant in crystals of two orders: the larger being very 
generally corroded and filled with intersections of the ground 
mass and particularly augite grains ; the plagioclase of the sec- 
ond order is in small, lath-shaped crystals, is less altered, and 
