Alfred Harker— On some Anglesey Dykes (No. III.) 269 
tremolite. Felspar crystals are plentiful, mostly once twinned and 
with moderately wide extinction-angles in sections perpendicular 
to the twin-plane. They are moulded and enclosed by large ophitic 
plates of augite with the characters usually found in that mineral in 
the Welsh diabases. The edges of these plates are touched here and 
there with brown hornblende, with the regular crystallographic 
relations to the augite, and possibly derived from it. Where ser- 
pentinous pseudomorphs are included in the augite, there is often a 
fringe of pale green or colourless amphibole, with a definite crystal 
orientation projecting into the serpentinous mass. This mineral is 
no doubt a “secondary enlargement’’’ of the augite crystal from 
which it grows, though its substance must be derived from the 
destruction of the olivine. 
Holyhead main dyke-—We now pass on to the island of Holyhead, 
where two large dykes were first noted and described by Henslow. 
The main one, with an average width of 60 feet, traverses almost 
the whole length of the island, running in a slightly curved line, but 
with an average bearing north-west and south-east, from near the South 
Stack to Cymmeran Bay. It cuts the quartz-rock, the green schists, 
and the serpentine, occasionally throwing off veins or branches. 
Henslow carefully notices the contact-alteration of the adjacent rocks, 
and makes the observation that “where the dyke is contained 
between parallel walls of the schist, and appears as though it were 
filling up some large crevice, the effects are never so striking as in 
those places where it ramifies and becomes intimately associated 
with the surrounding mass.”* He also remarks the variability of 
character displayed in this and the following dyke when traced 
along their length, a feature still more clearly brought out by an 
examination of slices under the microscope. 
[605.] Olivine-diabase from Porth-dafarch (Port Dafreth of Hens- 
low). ‘The name diabase is employed for this rock, despite a certain 
approach to the doleritic type, shown by a tendency to develope 
a second generation of felspars: the structure is ophitic. Olivine 
grains are abundant and tolerably fresh: they are sometimes slightly 
penetrated by the felspars. A few imperfect crystals of magnetite 
are included by the olivine, but the bulk of the mineral is posterior 
to the dominant felspar. The felspar, apparently labradorite, shows 
cross-twinning and some zonary shading in polarised light. A few 
rather shapeless crystals, with strong zoning and less pronounced 
twin-lamellation, belong to a rather later stage. Pale-brown augite 
forms large ophitic plates including olivine and felspars alike. The 
plates are divided by definite lines into distinct fields, with crystal- 
line continuity, but slightly different optical properties, and this 
structure sometimes approaches the regularity of the well-known 
‘“‘hour-glass augite.”’ 
[614.] Dolerite from a small branch of the dyke at the same 
locality. This rock presents no special peculiarities: if olivine has 
been present, it is now lost in ill-defined secondary products. The 
1 Gf. Van Hise, Amer. Journ. Sci. (3) xxiii. p. 385, 1887. 
* Trans. Camb. Phil. Soc, vol. i. p. 419, 1822. 
