264. Dr. F. H. Hatch—The Wicklow Greenstones. 
rocks caused them not unnaturally to be regarded and mapped as 
tuffs (“ greenstone-ash”’).! In the smaller sheets the foliation is 
very striking, the whole of the rock being converted into schist; 
the thicker bands, however, are foliated only at the margin, 
i.e. near the junction with the slates. 
About one mile and a half west of Woodenbridge one of these 
larger bands has been well opened out by a quarry ; and the suc- 
cessive stages of alteration admit there of easy study. The central 
portion of the sheet is not foliated. It is composed of a granular 
ageregate mainly of plagioclase, and hornblende, secondary after 
augite. The hornblende is the ordinary green variety (actinolite), 
and shows pleochroism in bluish-green and yellowish-green tints. 
Ji occurs in patches, which, though often compact towards the centre, 
are usually bordered by a fibrous fringe. These patches are pene- 
trated by needles and prisms of turbid felspar, thus indicating the 
ophitic structure of the original augite. In rare instances a nucleus 
of augite still remains, thus removing all doubt as to the secondary 
nature of the hornblende. The presence of isolated patches of chlorite 
shows that hornblende does not represent the final stage of alteration ; 
for the chlorite appears to be derived from the hornblende, and thus to 
bear only a tertiary relation to the augite. Other secondary minerals 
are epidote, in granular aggregates, leucoxene, in white turbid grains 
after ilmenite, and calcite. The epidote is derived mainly from the 
felspar, the original crystals of which are replaced by a mosaic of 
minute granules of secondary felspar, doubtless a variety less rich 
in lime—such as albite, or oligoclase. 
A fibrous asbestos is developed between the slickensided joint- 
faces. 
Near the junction with the slates the rock assumes a marked 
foliated character. In thin sections made from this portion of the 
rock the hornblende appears in narrow fibrous bands, alternating 
with layers of a cryptocrystalline aggregate of felspar together with 
grains of original felspar. Secondary quartz is associated with the 
new-formed felspar, and chlorite with the hornblende. The chlorite 
increases in proportion to the amount of alteration, until finally we 
get a highly fissile lustrous schist, in which chlorite greatly pre- 
dominates.’ 
Some specimens of these highly altered schistose rocks, for instance 
one from a band two miles west of Woodenbridge, contain abundant 
grains of sphene, and it seems probable that this mineral has been 
produced by a molecular reconstruction of the turbid leucoxene 
derived from ilmenite. 
Some of the greenstone-bands that cross the Aughrim Valley are 
1 According to Mr, W. M. Hutchings (this Macaztnz for February, 1889, p. 53) 
the same error has been committed in Cornwall, De la Beche having held certain 
‘«schistose trappean rocks,’’ which Mr. Hutchings has proved to be mechanically 
metamorphosed greenstones,’’ for altered ash. 
* Similar alterations are described by Mr. Hutchings in the Tintagel rock (/oc. cit.), 
and by Mr, Teall in a greenstone occurring at Garth near Portmadoe (Brit. Petrog. 
p- 216). 
