686 WHITMAN CROSS 
The isotropic constituent embraces most of what is megascop- 
ically visible, and also much more, in small grains, which, with 
corresponding ones of augite, magnetite, and feldspar, make up 
the groundmass. There is, in fact, a regular gradation between 
large and small isotropic grains. No crystal form was observed 
for the isotropic substance, but neither does it appear in any way 
to play the rdle of a glassy base. It seems throughout to be an 
irregularly granular mineral constituent, of the isometric system. 
The larger grains are almost wholly free from inclusions, and 
while probably the last substance to crystallize the isotropic 
mineral has in its growth pushed back the smaller grains of 
augite and magnetite so that they often form a distinct zone 
about it. This phenomenon seems clear evidence of a crystalliz- 
ing force. Thesmaller grains mingle with augite, magnetite, and 
feldspar. 
While no crystal form has been observed, rings or wreaths 
of small inclusions were noticed in a few grains, and these so 
strongly suggested leucite that until the chemical analysis was 
made I felt quite sure that the rock must bea leucite-basalt. A 
smoky tinge is present in a very few grains, and in one the col- 
oring matter is arranged in zones, clearly suggesting a regular 
crystal form. Irregular fractures traverse the substance, and its 
index of refraction is less than that of the Canada balsam, as 
indicated by Becke’s method. 
The feldspathic constituent appears in small, irregular, clear 
particles, some of which have most characteristic microcline 
structure, with an extinction of 15°; others can only be consid- 
ered as sanidine, with Carlsbad twinning in some grains; and 
the remainder is a plagioclase rich in soda, with very delicate 
albitic twinning. Its angle of extinction is always small. There 
is possibly some nepheline associated with the feldspar. 
The purity, freshness and abundance of the isotropic mineral 
invited the attempt to determine its composition by isolation and 
analysis. This was done by Dr. W. F. Hillebrand, and the 
result is given in column I of the table below. Under I|* is 
given the molecular ratio deduced from the analysis. The 
