OCCURRENCE OF CALCITE IN A BASALT. 281 
The calcite is granular, and filis the irregular interstices 
either as single grains oraS two or more grains. It is often 
twinned, and may show a kind of undulose extinction 
reminding one of a vague and ill-defined brush, such as is 
seen in spherulitic aggregates. The interstitial spaces 
filled by the calcite are up to 2°O mm. in length and are very 
irregularly bounded, quite unlike steam-cavities. Felspar 
and other minerals often project far into the calcite and 
are sometimes practically isolated by it (Plate XV, fig. 3). 
In places the calcite filling has perfectly sharp and straight 
boundaries against the felspar, but elsewhere the latter 
has been corroded, and partially replaced by the carbonate. 
All the circumstances here point to the calcite being of 
magmatic origin; the mineral plays an interstitial role just 
as quartz does in a granite, or the quartz-felspar inter- 
growth in a quartz-dolerite, or the primary analcite in an 
analcite dolerite, and, judging from its appearance in thin 
section, has as much right as they to be considered primary. 
Again, the fact that the calcite forms such an unusually 
high proportion of the whole rock means that without it 
the rock would be a mere network of crystals, with con- 
siderable empty interstitial space, a highly improbable state 
of affairs in a rock just consolidated. When we add to this 
the fact of the complete enclosure of certain of the crystals 
in calcite the conclusion seems inevitable that the latter is 
primary. Alternatively it might be suggested that the 
calcite replaces some other interstitial material such as 
glass, but there is no evidence of this either from the 
associated rocks, or in the shape of relics of replace 
material in the rock itself. 
The history of the rock-mass in its present condition 
would appear to be somewhat as follows:—After the con- 
solidation of the principal constituents as a network or 
sponge-like mass of felspar, augite, olivine and ilmenite, 
