270 Alfred Harker—On some Anglesey Dykes (No. IIT.) 
structure is distinctly porphyritic as regards the felspars; there is a 
rather fine-grained ground of ophitic appearance. 
[617.] Specimen from the southern part of the dyke, showing the 
junction of two different rocks. The coarser-grained one is seen 
under the microscope ta be a diabase, partly ophitic, partly granulitic 
in structure: no evidence of olivine is to be detected. 
The finer-grained rock has a ground-mass of minute felspar- 
microlites and crystals of magnetite, moulded by augite. In this 
ground are imbedded felspars of rectangular section and a few 
idiomorphic augites. This rock, which may be styled an augite- 
andesite, is doubtless a small dyke intruded into the diabase, for it 
shows a marked fluxional appearance along the line of junction. 
Cordier compared specimens from this main dyke of Holyhead 
Island with the “ granitoidal ophites of the Vosges.” 
Holyhead Eastern dyke.—The second large dyke in the island 
probably branches off at an acute angle from the main dyke about 
the middle of its length, and runs in a direction N.W. by N. past 
the east side of Holyhead Mountain, passing out to sea at the north 
end with a thickness of 80 feet. Our specimens are from the 
northern extremity, from Cae Seri on the road to South Stack, and 
from Bryniau Geirwen on the road to Porth-dafarch. They vary in 
grain and in general appearance to the eye, some resembling gabbro 
and others diabase and dolerite. Their intimate structure reveals 
corresponding variations, but it is perhaps best to class them all 
together as hornblende-diabases of various types. 
Under the microscope all these rocks are seen to contain a rather 
unusual quantity of apatite in colourless acicular prisms. Magnetite 
occurs plentifully, in composite crystalline forms, in ragged grains 
and patches, and in crystal frameworks. These two minerals 
always have the priority in the order of consolidation. 
The augite is of the very pale-brown variety, and when it exhibits 
crystal outlines, has the familiar octagonal cross-section. Besides 
the well-defined prismatic cleavage, there are traces of another, 
transverse to the length of the prism. The mineral is usually 
fresh; its chief secondary product seems to be serpentine. ‘These 
characters, which are approached by the augite of many Welsh 
diabases, agree with the variety malacolite. Hornblende is plentiful 
in all but one slide [686]. It is the deep-brown ‘basaltic’ variety, 
with absorption-formula y > 6 >> a. The mineral occurs some- 
times in good prisms, truncated, except in the smallest crystals, by 
the clinopinacoid. Much of the hornblende, however, occurs in 
close association with the angite, the two minerals having, as usual, 
the vertical axis and orthodiagonal common. ‘The hornblende 
mostly borders the augite, and when there are definite crystal out- 
lines to such a border, they are those proper to the former mineral. 
This is, therefore, not a case of ‘paramorphism,’ but an inter- 
growth of the kind termed ergdnzende or complementary hornblende. 
Patches of hornblende in the interior of the augite plates are 
probably also a parallel intergrowth. The hornblende crystals 
occasionally enclose grains of augite without any crystallographic 
