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Alfred Harker—On some Anglesey Dykes (No. III.) 271 
relation, and in all cases the hornblende in these rocks seems to be in 
the main posterior to the pyroxene. Greenish fibrous amphibole is 
common as a ‘secondary enlargement ’ in crystalline continuity with 
the original hornblende, and it sometimes extends as a fringe along 
the margin of the augite. A similar relation is met with in some of 
the German hornblende-diabases, e.g. the ‘proterobase’ of Ktirenz 
near Trier. 
Striated felspar is always abundant in the rock of the dyke, and 
appears to belong mainly to the andesine-labradorite series. It is 
often strongly zoned, or if the felspars do not all belong to one 
stage, the later ones show this structure. In accordance with the 
usual rule, the outer zones always give lower extinction-angles than 
the inner part of the crystal. 
The only other mineral to be mentioned is biotite, of which a few 
flakes are seen in one slide (Bryniau Geirwen). The specimens 
present then, it is evident, a community of general mineralogical 
constitution, and the diversity of structure noted below is therefore 
a matter of some interest. . 
[626.] From the northern termination of the dyke. In this rock 
the augite has idiomorphic boundaries, and is moulded by the felspar. 
The general aspect to the eye is not unlike some gabbros. 
A specimen of Mr. Marr’s from Cae Seri is a rather coarse-grained 
gabbro-like rock. The felspar and augite penetrate one another in 
a rather curious way, and must on the whole have consolidated 
simultaneously. 
[635.] Bryniau Geirwen. This shows the same peculiar type of 
structure ; the augite sometimes presents crystal outlines to the 
felspar, which moulds it, while in other places these relations are 
reversed, the felspar crystals penetrating the augite. 
[636.] Bryniau Geirwen. This specimen, from the same locality 
as the last, is an ordinary ophitic diabase. 
Finally, a rock procured by Professor Hughes from the same place 
is also ophitic in structure, but shows a decided tendency to the 
doleritic type. There are two generations of felspar; one in lath- 
shaped crystals penetrates the augite and hornblende; the other, 
more equidimensional and without regular outlines, is of later con- 
solidation than those minerals. 
The existence of such wide differences of structure in rocks forming 
part of one and the same igneous mass, offers a problem of which 
petrologists have not yet given a complete solution. 
Henslow collected specimens from two dykes, the larger one having 
a width of eighteen feet, at Port Newry, near Holyhead town. As 
he remarked, the rock here much resembles “the harder portions of 
the dyke at Port Dafreth” [605]. The chief difference is the greater 
abundance of olivine. 
[688.] Olivine-dolerite of Port Newry. The olivine, which often 
builds good crystals and has the pinacoidal cleavages well developed, 
is on the whole the first product of consolidation, though sometimes 
slightly penetrated, as at Porth-dafarch, by the earliest-formed 
felspars. The mineral is for the most part pseudomorphed by green 
