548 Alfred Harher — On some Anglesey Dykes. 



The constituent minerals of these rocks are apatite, ilmenite, 

 pyrites, magnetite, olivine, felspar, augite, hornblende, and biotite j 

 besides leucoxene, serpentine, calcite, quartz, chlorite, epidote, and 

 other secondary products. 



Apatite is often abundant in slender prisms, and seems to be 

 always the earliest formed constituent. 



Iron-ores are almost always among the original minerals, and 

 sometimes magnetite and ilmenite are recognizable in the same slide. 

 Ilmenite when best developed shows in skeletons of intersecting 

 rqds choked with grey leucoxene ; in other places the presence of 

 a titaniferous mineral can only be inferred from these cloudy grey 

 masses. Magnetite sometimes forms rods, but more usually imperfect 

 cubes. There is also secondary granular magnetite resulting from 

 the alteration of the iron-bearing silicates. In some of the original 

 grains a scarcely'' perceptible brown translucency may indicate 

 picotite. 



Olivine is rarely abundant, and for the most part not detected. 

 Occasionally rounded and oblong patches of serpentine show the 

 characteristic mesh-structure ; but the bulk of this substance met 

 with in the slides can be clearly traced to the decomposition of 

 pyroxene and amphibole minerals, and is usually mixed with chlorite. 

 The rocks east of Llanbabo, however, show plenty of olivine. 



Felspar of the lime-soda series seems to have been a universal 

 constituent, and locally abundant. In most of the slides this mineral 

 is replaced by calcite, chlorite, and quartz. Where the felspar is 

 fresh enough to exhibit its original characters, it seems from its low 

 extinction-angles to be never a very basic variety : it may be referred, 

 if this test is trustworthy, to andesine and oligoclase. Twin-lamel- 

 lation on the albite-law is almost always seen, but the pericline 

 twinning rarely. The felspar is usually later than the ilmenite, and 

 earlier than the augite and hornblende, which it penetrates ; but in 

 some slides from Llys Einion the felspar partly penetrates and partly 

 moulds the hornblende. 



Augite is not always present, but it is probable that this mineral 

 has been essential in all these rocks, and has been in great part 

 replaced by hornblende. In some slides from the Llys Einion and 

 Llanbabo dykes the process of conversion can be traced. The two 

 minerals have the orthopinacoids and the clinopinacoids parallel : 

 they are most intimately associated, with an extremely irregular line 

 of division, and the augite often remains as a core surrounded by 

 brown hornblende. 



Hornblende is the most abundant mineral in these rocks, and 

 occurs in several distinct varieties. It is the prevalence of one or 

 other of these varieties, as much as any essential diversity of con- 

 stitution in the rocks, that gives rise to the difference of aspect observ- 

 able in hand-specimens. 



Original hornblende is found in well-formed crystals, usually 

 from one-eighth to half an inch in length. The terminal planes are 

 probably in most cases the usual (Oil) and (lOl), but the basal 

 plane also occurs (001), and a very oblique one which maybe (201). 



