ROCKS 51 
meagrely as the case may be. Amongst the accessory minerals occa- 
sionally present, we may note biotite, hornblende, and hypersthene, each, 
when prominent, giving rise to a variety of the rock, as M/ica-, Hornblendc-, 
and Hypersthene-basalt. Olivine is sometimes wanting ; occasionally it 
occurs in rounded granular aggregates which may reach the size of a 
man’s head. Such aggregates are often rich in pyroxenes and 
spinelloids. 
Basalt is abundantly met with as an intrusive rock in 
the form of sills and dykes, and as an effusive rock or lava. 
Like all lavas, effusive basalt is often more or less vesicular 
and slaggy, the amygdaloidal cavities being lined and filled 
with such minerals as zeolites, quartz, chalcedony, calcite, etc. 
Tachylite is a basalt-glass which is sometimes smoothly homogeneous 
and compact, and at other times highly porous and vesicular. Basaé¢- 
pumice is either foam-like or spongiform, or may be drawn out in the 
form of hair-like threads containing long, cylindrical, gas _ pores. 
Tachylite sometimes occurs as a vesicular crust on certain basalt-lavas, 
the basal portions of which are also often highly vitreous. The same 
dark glass not infrequently forms the external surface of basalt-dykes— 
this “‘chilled edge” varying in thickness from a few lines to several 
inches. 
The diorites seem to be the plutonic equivalents of the 
effusive hornblende-, mica-, and quartz-andesites (dacites). 
These andesites, however, also occur intrusively. The 
effusive augite-andesites, and the basalts, on the other hand, 
being more basic, are closely related to the intrusive dolerites 
and gabbros. It will be remembered, however, that basalt 
often occurs intrusively, and the same is the case with augite- 
andesite. 
3. ROCKS WITH FELSPATHOIDS TAKING THE 
PLACE OF FELSPARS 
The only rocks belonging to this group that need be 
mentioned are Nepheline-basalt and Leucite-basalt, both of 
which are effusive rocks of relatively recent geological age, 
and having a rather limited distribution. 
Nepheline-basalt is black, and composed essentially of nepheline, 
augite, and olivine, with magnetite, apatite, biotite, and haiiyne as 
common accessories. Glassy base is occasionally present. Some 
varieties are as compact and fine-grained as typical plagioclase-basalt, 
from which in hand-specimens they can hardly be distinguished ; others 
