244 Transactions of the Royal Micrescopical Society. 
margins frequently indented by the base running far into the 
crystals, The quantity of magnetite grains in this sliced specimen 
is unusually large, and imparts a very dark character to the 
base. 
General Remarks on Microscopic Structure. 
The examination of this series of lavas, and the lavas of the 
district in general, proves conclusively that the minutely crystalline 
base of small felspar needles is the prevalent form, though fre- 
quently much obscured by subsequent alteration. These needles 
are undoubtedly in many cases triclinic felspar, but in some they 
may be orthoclase twins of the Carlsbad type. The larger crystals 
are generally plagioclase, though orthoclase seems to occur some- 
times. ‘They are replaced both by chlorite and serpentine, but 
owing to the great number of pseudomorphs of green minerals 
after both augite and felspar, it is by no means always easy—unless 
distinctly aided by the crystalline form—to distinguish between the 
various original constituents. Some of the chloritic pseudomorphs 
have curious little yellowish granules disposed in irregular lines 
along them, as seen in Fig. 2. In yet other cases the replacement 
has been such that in polarized light the whole of the crystal 
shows a mosaic of different colours, as seen in one case in Fig. 4. 
The many examples in which the porphyritically imbedded 
crystals either contain portions of the base shut up within them, or 
show it running far into their sides, like inlets and bays, clearly 
indicate that such crystals were formed previously to the consolida- 
tion of the base. The frequent enclosure of grains of magnetite 
within the glass cavities of the augite also points to the same fact, 
and to the very early crystallization of the magnetite. Indeed, the 
order of formation would seem to be, first the magnetite, next the 
larger crystals of augite and felspar, and lastly the base, so largely 
composed of minute needles of felspar, which frequently assume a 
well-marked flow around the larger crystalline masses. 
With regard to the augite crystals, it is striking how many 
slices may be examined, and yet none but pseudomorphs after the 
mineral be met with, while in other cases—though rather ex- 
ceptional— most of the larger crystals appear almost unaltered, and 
frequently exhibit well-marked twins. If this alteration be so 
prevalent among the larger crystals, there is but little wonder that 
the finer augitic matter which probably once existed throughout 
the base, in many or most cases should have been all completely 
changed, its place being taken by chloritic minerals. In Fig. 4 
examples may be seen of partially altered augite crystals, under 
polarized light. 
The great quantity of pseudomorphic chloritic matter which 
prevails among these old lavas entitles many of them to be classed 
