Calcite from Luganure. 335 
says that the faces are dull and somewhat rough, asis frequently 
the case with those prisms [les faces en sont mates et un peu 
raboteuses, circonstance fréquente pour le second prism (7. e. d') 
i six faces”]. The difference in lustre between the faces of the 
two kinds of prisms is characteristically seen in the dodecagonal 
prisms (chaux carbonatée péridodécaédre of Haiiy), which is 
the combination oR, oR, oP (d' e a'); the faces oR (e?) are 
always very much more brilliant than oP (d'). This difference 
of lustre is one of the distinctions relied upon to distinguish the 
’ faces of the two kinds of hexagonal prisms from one another. 
Dufrénoy also notices this difference between the two kinds of 
prismatic faces in the twelve-sided prisms. 
Several forms of the rhombohedral prism occur at the Lu- 
eanure mines, County of Wicklow, which are worked for galena 
in a veinstone consisting chiefly of quartz, in a granite country. 
Among these may be mentioned oP, oR (d', a'), consisting of 
small hexagonal prisms, with very bright prismatic faces. One 
half of the prism is hyaline, and the other opalescent ; the base 
oR is dull. Another variety of the same form also occurs, con- 
sisting of crystals one centimetre high, and with basal edges one 
centimetre long. Each crystal has a sort of rude triangular 
prismatic milky nucleus, surrounded by a perfectly hyaline enve- 
lope, reminding one of the description of Haiiy given above. 
Owing to the number of cleavage planes, some crystals are not 
transparent. The face oR is in most instances peculiarly 
striated, in others it is, as it were, coated with a thin porcela- 
neous layer. These crystals may be easily cleaved parallel to 
the alternate basal edges, which are sharp, and without any 
trace of modifying planes. The form oR, oR (a', ¢*) also occurs 
in beautiful hexagonal plates, with very bright prismatic faces, 
and composed of exceedingly thin alternating layers of white 
opake, and hyaline matter, the base oR being always opake, 
dull, but beautifully white. Haiiy’s description of the prismatic 
kind embraces this variety likewise; in fact, the specimens 
from Luganure here described illustrate perfectly Haiiy’s de- 
scription. 
I have lately, however, met with another form, consisting of 
hexagonal plates, of from one millimetre to one and a half thick, 
with basal edges of from five to twenty millimetres. The base 
has a bright nacreous lustre, much brighter than what I have 
ever seen in any other specimen; striated and uneven, in conse- 
quence of the lapping of smaller plates. The most of the tabular 
prisms are, in fact, compound twins to the base oR (a'). Some 
twins also occur to the faces of the prism, and finally, to a 
rhombohedron. It is owing to this twin structure that the 
crystals are not generally transparent ; for in thin plates they are 
