Alfred Harher — Some Anglesey Dykes. 415 



extinctions, of a more acidic species. These are untwinned, and 

 show the zones of growth very clearly. They are without crystal 

 boundaries, and sometimes include little crystals of the older genera- 

 tion. Magnetite occurs in small cubes of later formation than the 

 first set of felspars : it is not very abundant. The augite in pale- 

 brown patches moulds or completely includes the felsjDars of the 

 earlier generation. In some places it is crowded with short black 

 rods, probably of magnetite, disposed parallel to two definite direc- 

 tions : this appears to be a secondary phenomenon connected with 

 the decay of the augite. The usual feebly polarizing ' viridite ' 

 patches replace this mineral, and there is some calcite dust in the 

 «lide. The infilling of the vesicles has taken place in several 

 distinct stages. First a zeolitic mineral has been deposited at points 

 on the wall, in fan-like bundles of imperfect crystals. The interior 

 of the cavity, thus reduced in size, has been stained by a greenish- 

 yellow substance, and then lined with chalcedony in the usual 

 mamillary coating. The remainder of the vesicle has finally been 

 filled with calcite, not in one mass, but as a mosaic of distinct 

 crystalline grains, mostly untwinned. In some cases a little clear 

 quartz occurs with the calcite. 



Llanddwyn dykes. — Henslow collected specimens from two dykes 

 in the island or peninsula of Llanddwyn, westward of the opening 

 of the Menai Straits. They do not agree in position with those 

 marked on the Survey Map. The rock is much decomposed, and 

 only one slide has been pi-epared. 



[684.] Porphyi-itic dolerite from Llanddwyn. — This is a fine- 

 grained, much- weathered rock inclosing liver-coloured felspars more 

 than an inch in diameter. One of these is shattered, but the parts 

 still remain in proximity to one another. The microscope shows 

 abundant cubes of magnetite, small felspar prisms much altered, and 

 augite replaced by the characteristic pale-green product with some 

 calcite. There is a small vesicle filled by a deposit of quartz 

 followed by calcite with polysynthetic twinning. The large porphy- 

 ritic felspars are deeply affected by an apparently saussuritic altera- 

 tion. In some places, however, they are clear enough for their finely 

 repeated twinning to be made out, and the extinction-angles are 

 such as would be given by labradorite. Similar felspars occur in the 

 Glan Adda dyke already mentioned. 



All the dykes of the Menai Straits, with perhaps the exception of 

 the olivine-bearing rock of Plas Newydd, are intermediate rather 

 than basic in their affinities. The characters of the augite are in 

 accordance with this ; so also the frequent cross-twiuning in the 

 larger felspars and their very pronounced zones of growth, the 

 abundance of magnetite to the exclusion of ilmenite, and the nature 

 of the ground-mass where it occurs. The rocks are all ' ijorphyritic ' 

 in the sense of Eosenbusch, since they contain felspars of more than 

 one generation. When the later felspars occur as microlites forming 

 part of a ground-mass, I have used the name augite-andesite. Eocks 

 of more holocrystalline type, in which the two sets of felspars are 

 of about equal size, have been called dolerites, as distinguished from 



