412 Alfred Harker — Some Anglesey Dykes. 



second generation, and sometimes sliows crystal-outlines, the prism 

 and pinacoidal planes. Iron-ores are abundant, chiefly magnetite ; 

 fine needles of apatite occur in the felspars ; small scattered flakes 

 of biotite are sparsely present. The latter mineral is met with in 

 the Cadnant dyke [525], "a circumstance of rare occurrence in the 

 dykes of Anglesea," (at least in those of the Menai Straits). The 

 Glan Adda dyke has pink porjihyritic felspars, which under the 

 microscope show large squarish sections, much decomposed. 



It will be noticed that all these dolerites correspond in character 

 more closely to the andesitic than to the basaltic family : olivine I 

 have not yet observed in the above rocks, though it may possibly be 

 lost among the decomposition-products. This mineral occurs, how- 

 ever, in the Plas Newydd dyke described below. 



[546.] Dolerite : near Four Crosses, on the north-western pro- 

 longation of the Cadnant dyke. — This specimen, to illustrate the 

 more compact type of rock, is taken from the outer portion of the 

 dyke, though not from the actual contact, where the andesitic habitus 

 prevails. The magnetite is mostly older, but partly newer, than the 

 felspar ; it occurs in crystals showing the cube or octahedron, and in 

 complex shapes related to those forms. The minuter grains often 

 appear in star-like aggregates with sections exhibiting three bilobate 

 Jays : these are sometimes surrounded by a ring of finely granular 

 magnetite. The felspars give elongated rectangular sections : the 

 larger ones are often simple or once twinned, but fine twin-lamellation 

 is also found. The pale-brown augite, in ophitic plates of varying 

 extent, is mostly destroyed. As secondary products occur the usual 

 " viridite," finely granular calcite, and clear quartz. 



The dykes in the neighbourhood of Menai Bridge have not been 

 sliced. They are dolerite, often with porphyritic felspars. They 

 a})pear to be much decomposed, and contain a considerable amount 

 of calcite and pyrites, 



Plas Neicydd dylce. — The largest dyke in this part of the island 

 cuts through the lower beds of the Carboniferous Limestone series a 

 little south of Plas Newydd, the seat of the Marquis of Anglesey, 

 This dyke, 134 feet wide, occupies a probable line of fault bearing 

 in a south-easterly direction : it is met with again on the opposite 

 side of the Straits. In the nomenclature of Cordier's day the rock 

 was described as " indubitable basalt, consisting of felspar and 

 pyroxene," but the normal type is holocrystalline and granular. 



[485.] Olivine-Dolerite ; typical rock of the Plas Newydd dyke ; 

 a rather coarse-grained dolerite with marked ophitic structure. — The 

 microscope reveals olivine in abundant rounded grains included in 

 the augite. There is much secondary magnetite dust resulting from 

 its decomposition. Magnetite occurs also in crystals, usually imper- 

 fect cubes ; this and the olivine are the earliest formed constituents. 

 The chief felspars are in elongated crystals showing finely repeated 

 twinning and often concentric zoning; the extinction angles are 

 moderately high, and would agree with labradorite. There is some 

 slight bending of the crystals, which may perhaps affect the twin- 

 lamellation. Sometimes cross-twinning, presumably on the pericline 



