Vol. 58. ] JASPERS OF SOUTH-EASTERN ANGLESEY. 427 
and is thus confined to the limits of what is a crystalline or optical 
unit. The mottled texture is therefore, now, a mineral and not 
an organic structure; but it is of course quite possible that it may 
have been originally due to the presence of organic bodies the 
outlines of which have become obliterated. (See Pl. XV, fig 1.) 
The spherulitic varieties, when best developed, are composed of red 
spherulites, 2 to 3 millimetres in diameter, in a darker matrix. The 
spherulites have cclourless, granular cores, around which is the pale 
red body, with radial structure, giving a dark cross between crossed 
nicols. The intervening matter is very dark with iron-ores, often 
sufficiently well crystallized to show definite flakes of ‘ eisenglim- 
mer, and evidently much altered. No organic bodies have been 
found in the cores of the spherulites. 
The texture of the ordinary jaspers varies considerably within 
the limits even of a microscopic slide, some parts being rather 
coarser, and some more free from hzematite than others. Numerous 
veins of quartz traverse the rock in all directions. These are of 
much coarser texture than the fine mosaic of the main body, and 
are also generally much more free from inclusions. A slide from 
Fferam-gorniog, near Pentraeth, contains also some curious groups 
of small doubly-terminated crystals of quartz. Well-formed rhombs 
of calcite or dolomite are not uncommon in the fine mosaic, and to 
a less extent also in the quartz-veins. 
The jaspery phyllites have the same general appearance as 
the jaspers, but have a parallel structure and a duller surface. In thin 
sections they appear, like the jaspers, to be composed chiefly of very 
fine granular quartz, and hematite, but there are also present 
numerous minute clastic grains of white mica, and also of quartz: 
and the general texture is that of a fine ferruginous shale or mud, and 
unmistakably sedimentary. (See Pl. XV, fig. 2.) 
(ii). The igneous rocks are diabases and serpentines. 
_ The serpentines are of small size, and have only been observed 
in one or two places in the Pentraeth area. Though the fact of their 
existence is of considerable interest, no petrological detail concerning 
them is necessary for the present subject. 
The rest of the igneous rocks are more or less altered dolerites 
or basalts, generally fine-grained, and, from their lightish tint 
when weathered, evidently not very basic in composition, They 
are generally of a dull green, but when slightly deformed are often 
reddish, and their schistose parts can then be easily mistaken for 
the red phyllites associated with the jaspers. Thin slides show 
a fine meshwork of slender lath-felspars, with some iron-ores, and 
sometimes (in the Newborough district) cores of augite still 
remaining. Generally, however, the pyroxenes have been replaced 
by the usual green alteration-products. Sometimes a tendency to a 
sheaf-like or radial aggregation of the felspars is well marked. 
(pee. PI. Vi tie. 1.) 
But the most remarkable characteristic of these rocks is their 
structure on the Jarge scale. In the field they are seen to be 
