300 Scientific Proceedings, Royal Dublin Society. 
a narrow central bridge: this and the proximal part of the lobes 
consists of an irregular mosaic of quartz grains, while the remaining 
distal part of the lobes is composed of fibrous quartz, the fibres of 
which proceed from the granular mosaic outwards to the margins 
of the lobes, lying in close parallelism to the cleavage planes of the 
slate. The distinction of the fibrous from the granular quartz is 
clearly marked, and is sometimes emphasised by the presence of 
minute black granules, apparently of iron pyrites, which are dotted 
along the line of junction. Similar granules not infrequently occur 
arranged in circles within the fibrous quartz, as though surround- 
ing spherical growths of chalcedony. Pyrites in larger or smaller 
crystals, sometimes partially converted into limonite, may take the 
place of silica, over any part of the section, without as a rule dis- 
turbing its form (fig. 2, ¢, e, f). 
A want of symmetry is not uncommon in the sections. The 
upper and lower margins of the lobes may be unequally curved, 
and then it usually happens that the are of greater curvature lies 
on the upper side of one lobe and on the lower side of the other. 
When the inequality is very marked the granular quartz extends 
into each lobe along opposite margins (fig. 2, c). A bifurcation of 
the lobes is sometimes to be observed as shown in fig. 2, e. A still 
more extreme case is represented in fig. 3. 
Fie. 3. 
Transverse section, showing bifurcation of the lateral lobes (x 60). 
The form presented by the transverse sections is probably not 
original. ‘The threads have been squeezed flat in the planes of 
cleavage ; and to discover the primitive form we should seek for the 
transverse section of a thread running across the cleavage planes. 
I have not, however, found completely satisfactory evidence of 
such sections. Those which seemed to be transverse were oval in 
outline, with a granular centre and fibrous margins, the fibres 
running parallel to the long axis, and being most abundant about 
its extremity. ‘There is just a possibility that these are disjointed 
fragments of longitudinal sections ; if they re not but are actually 
