488 T, W. E. DAVID, W. F. SMEETH, AND J. A. SCHOFIELD. 
forming the surrounding shells. The rock isa fine grained basalt 
composed of a felted mass of felspar microlites with minute grains 
of greenish augite, magnetite, and brownish-red olivines. The 
presence of any glassy material is doubtful. The texture is very 
fine and quite uniform, no phenocrysts having been met with. 
All the basalts described so far bear a striking resemblance to 
each other, both in general characters and in mineralogical con- 
stitution. Their differences consist in the varying amount of 
glassy base and slight changes of texture, some having a few por- 
phyritic constituents, and some being vesicular. 
_ The small brownish-red grains of olivine form a peculiar feature 
common to all. Were it not for these olivines, the general 
characters of these lavas, and the fact that most of the felspars 
indicate a composition not lower in the series than labradorite, 
would suggest that they should be placed among the basaltic 
andesites. On the other hand the olivine is not present in suffici- 
ent quantity to make them olivine-basalts. In the absence of 
chemical analysis their exact classification is not ascertainable, 
and we have therefore left them with the somewhat indefinite 
designation of basalt. 
Olivine Basalt, (No. 10). 
A dark scoriaceous rock with numerous large olivines and 
black crystals of augite. Under the microscope the ground-mass 
is fine in texture, containing a number of very minute lath-shaped 
felspars. With higher magnifying power the base is seen t0 be 
glassy, slightly greenish-brown in colour, with numerous globulites 
and highly refracting grains. Intermediate in size between these 
and the felspars are numerous grains of pale brown augite, and 
also of brownish-yellow olivine. These olivines have a granulat 
appearance. There are also many small magnetite grains. 
Coming next to the porphyritic minerals we have :—Larp? 
crystals of magnetite. Large rounded grains of olivine, which 
are colourless but turning to a brownish-yellow along the crac™ 
Finally, very large crystals of augite. These have idiomorphi¢ 
j 
