572 Scientific Proceedings, Royal Dublin Society. 
The length of one of these lenticles depends on whether it has 
resulted from the elongation of a single crystal, or of a glomero- 
porphyritic group, being naturally longer in the latter case. The 
final result is the production of a banded rock composed of alter- 
nating dark (biotite) and light (felspar) laminz which presents, on 
a minute scale, the characteristic structures of a banded gneiss 
(fig. 4, Pl. xxvit.) 
Under the microscope the first sign of the effects of pres- 
sure in the felspars is the production of strain shadows, which is 
followed by fracture and strain slip cleavage. A. continuance of 
the pressure results ina certain amount of granulation, after which 
the lengthening process goes on apparently by solution and recrys- 
tallization to the final stage. In this the original crystal of 
plagioclase is reduced toa granular mosaic the individual grains of 
which show little or no strain shadows, being largely re-crystal- 
lized material, consisting of water-clear quartz and felspar, with 
biotite, and some secondary muscovite. Accompanying these are 
secondary crystals of colourless epidote and zoisite. 
In this condition the ground only differs from the sheared 
crystals in the relative abundance of secondary biotite, and the 
original rock has been reduced to one consisting of practically 
uniform-sized grains, a rather characteristic structure of gneisses 
and schists. 
The biotites are all orientated parallel to the direction of 
shearing, except where they occur between the felspathic lenticles. 
In these positions they often stand in a direction at right angles to 
the direction of shear. It is very remarkable that in no instance 
are the ends of two adjoining sheared lenticles of original felspar 
welded together, so to speak, by the mashing action which the rock 
has undergone. ‘They are invariably separated by a thin film of 
the matrix (fig. 4, Pl. xxvit.), and nowhere, so far as can be seen, 
has any mechanical mixture of the felspar and the ground been 
produced, both maintaining their separate individualities to the 
end. ‘This seems to show that the rock at all times moved practi- 
cally as a solid, and never approached the molten condition. 
The ground of the rock, however, which started practically in 
the condition as regards texture to which the plagioclase pheno- 
crysts were eventually reduced, in turn produced new structures 
while these phenocrysts were being stretched out. The one most 
