268 Alfred Harker—On some Anglesey Dykes (No. III.) 
rather pronounced type: the specimen is from the Henslow 
collection. 
[571.] Dolerite from dyke in coal-pit at Llanfihangel. This rock 
is very fresh and of rather coarse grain: it shows at a glance a 
tendency to parallelism in the felspars. Magnetite is abundant in 
the slide, partly in square sections earlier than any other constituent, 
partly in granular and imperfectly crystalline patches later than the 
dominant felspar, but included in the augite. 
The felspars divide into two distinct generations. The earlier 
ones occur in well-formed crystals, of elongated rectangular section 
up to O-1 inch in length, with a fine distinct twin-lamellation. 
These lamelle, sometimes interrupted or discontinuous, correspond 
to the albite law, but they are often crossed by pericline twinning, 
and some of the larger crystals exhibit the Carlsbad type also. 
These earlier felspars have little or no zoning, and seem to be 
andesine or labradorite. They are, however, not strictly all of one 
stage, for we may see one crystal moulding another, which in turn 
encloses a small felspar of still earlier consolidation. 
The later generation of felspars differs considerably from the 
former. The crystals are less elongated and usually shapeless. 
They are less finely twinned, and the lamellation is rendered indis- 
tinct by the strong zonary banding which is made manifest between 
crossed Nicols. In all these Anglesey dolerites this growth in con- 
centric zones of different optical characters is highly characteristic of 
the later felspars. It points to a rather rapid change in the chemical 
constitution of the magma during the closing stages of consolidation, 
and perhaps depends partly on the almost simultaneous separation 
of the augite. 
In the present rock the augite is not very abundant, and does not 
build ophitic plates of any great extent. It rarely presents imper- 
fect crystal boundaries to the later felspars, and is on the whole 
nearly contemporaneous with them. The mineral is pale brown, 
with the usual prismatic cleavage. 
It will be observed that the Llanfihangel rock compares closely 
with those from the Straits, such, for example, as the Cadnant dyke 
[545]. 
Before proceeding further, it may be noticed that there are other 
dykes in southern Anglesey, which are probably not to be grouped 
with the foregoing. They differ from them both mineralogically 
and structurally, and approach more nearly to the characters of 
parts of the Holyhead dykes to be described below. Asan example, 
a dyke at Bodowen, on the west side of the Malldraeth estuary, is 
chosen. 
[598.] Olivine-diabase of Bodowen. This is an ophitic rock with 
but one set of felspars. Under a low objective we see first skeletons 
of ilmenite, clouded with semi-opaque leucoxene, and some crystals of 
magnetite. There are also numerous rounded light-green patches, 
in part serpentine, in which are imbedded granules of unaltered 
olivine. These patches, often included in the augite, are crowded 
with minute matted fibres, perhaps chrysotile, but more probably 
