410 Alfred Sarker — Some Anglesey Dykes. 



evident. The faces which give the felspars their tabular habit are 

 the brachypinacoids, while the other forms present may be the 

 macropinacoid and basal planes. The usual albite-twinning is seen, 

 combined with the Carlsbad type : the extinction-angles seem to 

 indicate labradorite. The crystals are often bent and partially 

 fractured, but I have not observed any relation between these dis- 

 turbances and the twin-lamellation. Besides the felspars are abun- 

 dant crystals of magnetite ; and the ground-mass is composed of 

 felspar microlites, magnetite, and rounded granules of augite 

 decomposing into a pale-green substance. In both the ground-mass 

 and the larger constituents, the magnetite seems to be of rather 

 earlier formation than the felspar. 



[642.] Augite-andesite. — Henslow mentions this rock as " more 

 remarkably fine-grained and tough than any other which I have met 

 with in Anglesea." It is a contact specimen with a fragment of the 

 green schist firmly adhering to it. Here the scattered felspars are 

 smaller and occur more sparsely ; they show little of the tabular 

 habit and parallel disposition so striking in the preceding slide. 

 The ground-mass too is a little finer ; but with these exceptions the 

 same description will suffice. 



To illustrate the modifications of texture in different parts of one 

 dyke, two specimens collected by Prof Hughes have been sliced, 

 They are from a dyke just south-west of Gallows Point. 



Augite-andesite from marginal portion of dyke. — This is inter- 

 mediate in texture between [640] and [642], and has the same 

 general chai'acteristics. The scattered felspars, however, have inclu- 

 sions of magnetite, disposed chiefly along the twin-planes, 



Dolerite from central part of the same dyke. — The slide exhibits 

 rectangular sections of felspar, mostly lath-shaped and about 0-1 inch 

 in length, magnetite in crystals giving quadrangular sections and in 

 rods, and decomposed augite in ophitic plates. There is no ground- 

 mass. The felspars show twinning by different laws. In some 

 cases a crystal is divided by a line which may represent Carlsbad 

 twinning into two halves, of which one shows lamellation on the 

 albite and the other on the pericline type. The lamellge are often 

 interrupted. 



Another of Prof. Hughes' specimens, from a dyke north-east of 

 Garth Ferry House, may be styled a porphyritic dolerite. — This 

 rock bears a close resemblance to the preceding, but differs from it 

 in the fact that the magnetite occurs in imperfect crystals of later 

 formation than the felspar. Besides the rectangular felspars there 

 are a few of larger dimensions giving squarish sections. This rock 

 shows more advanced decomposition than the others mentioned 

 above. The augite, which appears to have formed ophitic plates, is 

 entirely destroyed and replaced by the usual pale-green substance 

 with confused scaly structure, and polarizing in low tints of grey 

 and indigo. Patches of this ' viridite ' inclose crystalline grains of 

 calcite. Grains of secondary quartz are also plentiful, as well as 

 magnetite dust and ferruginous specks. The felspars are almost 

 opaque. 



