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vidual minerals show the effects of strain to a considerable 
extent. Decomposition has progressed very slightly indeed. 
Quartz is abundant in perfectly clear and very much 
rounded sections of comparatively small size. In the last 
rock described quartz formed a mosaic of small grains, but 
in this ease there is nothing that can be called a mosaic. The 
individual grains are comparatively widely separated and are 
perfectly independent optically. Тһе mineral is sparingly 
dusted through with very minute inclusions, which, under 
the highest power available, appear to be filled, some with 
liquid, some with gas. Тһе shapes of these cavities are mostly 
irregular, but polygonal forms (negative crystals) are by no 
means rare. The liquid cavities have bubbles, whose volume 
bears no fixed ratio to that of the liquid. Movement of the 
bubbles is very pronounced. In addition to these unindi- 
vidualised inclusions the quartz grains contain small grains 
and crystals of biotite, zircon, and rutile (?), the last two 
very rare. 
Felspar is abundant in relatively large, perfectly allotrio- 
morphic grains, the outlines of which are often very angular 
and irregular. At first sight there seems to be no connection 
between adjacent grains, but closer inspection shows that, 
in a fair number of instances at any rate, two or more neigh- 
bouring granules are certainly parts of a single original erys- 
tal. All stages between slight difference of optical orienta- 
tion and a complete obliteration of all trace of relationship 
are met with. As a rule, the mineral is nearly free from 
decomposition products. Where the latter are present in any- 
thing like considerable quantities they are generally thickest 
along the (010) and (110) cleavage planes. In many in- 
stances the felspar is so perfectly water clear that it would 
be difficult to distinguish it from quartz but for the twinning, 
or, in its absence, the refractive index. Whilst mechanically 
the effects of rock movement are very marked in the break- 
ing of the grains described above, opticallv the felspar is 
singularly free from any sign of strain. In all cases where 
the section is not parallel to the plane of composition fine 
lamellar twinning after the albite law is beautifully developed. 
Carlsbad twinning is associated quite often, but in only one 
instance have I detected lamellæ after the pericline law. In 
an odd crystal here and there the lamellæ are faulted, but 
they are nowhere bent, nor is there any extensive undulose 
extinction. This combination of characters makes the opti- 
cal determination of the species easy and satisfactory. Sec- 
tions in the zone perpendicular to (010) give maximum sym- 
metrical extinctions of 13^. The difference in the extinction 
of different parts of the Carlsbad twin is just perceptible with 
