308 STRUCTURE OF ROCKS FROM ANGLESEY. 
which suggest that when found deposited they were slightly rounded 
grains. It is much decomposed, in many places full of minute mi- 
croliths and earthy dust, but the striation of the plagioclase group 
ean here and there be distinguished. The chlorite occurs in filmy 
or fibrous-looking scales, of no very definite shape, but more like 
mica than any thing else. The third mineral is in angular and club- 
like grains ; from the outline, ilmenite or hematite seem most likely. 
It is difficult to say what the rock has formerly been, but it is such 
a rock as I should expect would be formed from the coarse detritus 
of a dolerite, subsequently metamorphosed. 
XI. Railway-cutting near Ty-Croes ae 303). 
A very characteristic gneiss consisting of quartz, felspar as in VI., 
one or two grains much resembling microcline, and associated chlo- 
rite, opacite, and a colourless mica (probably replacing biotite); in 
the felspar are a good many irregular microliths of secondary for- 
mation (? some of them epidote), and a few others, perhaps apatite. 
XIL. Quarry near the Anglesey Column (p. 304). 
A feliated dense felted mass of a dull greenish, rather decidedly di- 
chroic mineral (probably a species of chlorite), and of small greenish- 
yellow epidote crystals, with a few angular fragments of quartz (?) 
and two or three scales of mica (? paragonite). 
(For Discusston, see page 325.) 
