MUSEUM OF COMPARATIVE ZOOLOGY. 171 
ger, “ Die Sil. Etagen 2 und 3,” etc., p. 261). In the rock as a whole 
the same even balance between Na and K is noticeable, and the insig- 
nificant quantity of lime and magnesia. Allowing for the free quartz, 
base, and decomposition products as causing a relative increase of silica 
and iron and decrease of the alumina and alkalies, it is evident that the 
feldspars of the groundmass are closely allied chemically to the porphy- 
ritic crystals, and are probably also anorthoclase. The rock is therefore 
a very pure type of keratophyre. 
The microscopical structure of the sections made are as follows : — 
No. 21. Keratophyre with anorthoclase crystal cut obliquely to an optic 
axis. Groundmass made up of minute twinned lath-shaped crystals of feld- 
spar, somewhat kaolinized, some quartz, and an earthy fibrous kaolinized base. 
In the centre of the porphyritic feldspar crystal are numerous microliths and 
a few ferritic masses, similar to and probably composed of the base, which 
penetrates the edges of the crystal. 
No. 21 A. Keratophyre and an aggregate of the porphyritic crystals. 
Groundmass nearly as in No. 21. One of the phenocrysts shows twinning 
after the Carlsbad type. 
No. 21 B. Keratophyre with one porphyritically enclosed crystal. The 
erystal is cut nearly parallel to the second cleavage, and gives an almost per- 
fect interference figure of the positive bisectrix. The basal cleavage is well 
developed, and the striz, or fine twinning, are well marked in polarized light. 
The groundmass is more generally composed of the minute lath-shaped feld- 
spar crystals, some of which are clearly twinned anorthoclase of the same form 
as the larger crystals. There are also small patches of quartz. 
No. 21 C. Keratophyre with one large porphyritic feldspar crystal cut 
obliquely to the brachydiagonal, which in polarized light shows a microper- 
thitic intergrowth and a very perfect example of fine and interrupted twinning. 
Through the crystal are several fluid cavities and a few microliths of a reddish 
color. The groundmass is more kaolinized, and the minute lath-shaped crys- 
tals are less distinct. Small irregular masses of quartz and considerable 
limonite and earthy matter pervade this section. 
No. 21D. Keratophyre section cut across a joint plane which is filled with 
vein quartz ; numerous irregular patches of quartz are scattered all through 
the section. One mass is a basal section of original (?) quartz ; it gives the 
uniaxial cross, and is shown to be positive by the mica plate. Some scales 
of biotite and numerous small grains of magnetite are seen in the groundmass, 
which is composed of a fibrous feebly polarizing kaolinized mass of the decom- 
posed minute lath-shaped feldspar crystals. One of the enclosed phenocrysts 
cut nearly parallel to the base shows numerous microlithic inclusions, and sev- 
eral fluid cavities in which the bubble movement is seen The outer edge is 
deeply penetrated by the groundmass 
