Alfred Harker — On some Anglesey Dykes. 547 



name of Hornblende-picrite. It was subsequently pointed out by 

 Professor Hughes that the probable source of these rocks was to be 

 found in certain inti'usive masses near Llanerchymedd, and indeed 

 such boulders are scattered about rather abundantly in that neigh- 

 bourhood and to the south-west. The rock in question seems, 

 however, to be the common type of the larger eruptive masses in 

 the north of Anglesey, and brief notes on slides cut from selected 

 specimens taken in place may be found not unprofitable. The rocks 

 were noticed and megascopically described in Henslow's Memoir.^ 



There are three localities in each of which similar varieties occur. 

 The first lies to the north and north-east of the town of Llanerchymedd, 

 where the map of the Geological Survey shows nine intrusions. 

 These masses, not properly dykes, have an oval or elongated form, 

 with lengths of from 200 yards to three-quarters of a mile, their 

 long axes running generally with the strike of the adjacent strata. 

 These beds, Arenig according to Professor Hughes,^ are baked at 

 the junction, but not extensively altered. On Henslow's map no 

 attempt is made to separate the several patches, which are marked 

 as one area of " greenstone." I have collected specimens from the 

 six masses lying between Ehodogeidio and Llandyfrydog, but am 

 not able to state whether the three patches to the north of the latter 

 place are of similar characters. 



A large irregularly-shaped mass appears on the western slope of 

 Llaneilian Mountain, in the north-eastern corner of Anglesey. This 

 has been mentioned by Professor Hughes and examined micro- 

 scopically by Professor Bonney. It too pierces rocks probably of 

 Arenig age, as well as the debatable Amlwch series. Our specimens 

 are from Pengorphwysfa. 



The third area is situated to the east of Llanbabo, among the 

 alluvial deposits of the Eiver Alaw. The exposures here are, 

 according to Henslow, " not well exhibited," and they have escaped 

 the notice of the Survey ; but the locality is verified by Professor 

 Hughes, who obtained a similar rock from south-west of Glas-grug. 



Kocks having many points in common with these occur as parts of 

 some large dykes in Holyhead Island; but they will be more con- 

 veniently considered apart, and our present description will apply 

 only to the three groups of intrusive masses mentioned above. The 

 specimens are selected as follows : — 



I. Hafod-in-in dyke (A 130, 131) ; 

 Ehodogeidio dyke (A 132) ; 



Llys Einion dyke (A 86, 91, 94, 134, and Hughes Coll. 33) ; 

 Maen Chwynt dyke, probably (Sedg. Coll. 37) ; 

 Llandyfrydog dyke (Henslow [695]) ; 

 II. Llaneilian Mountain intrusive mass (A 35, 37, and Hughes 

 Coll. two slides), all from Pengorphwysfa near Amlwch ; 

 also Henslow [715] ; 

 III. Dykes or masses east of Llanbabo (Henslow [711, 712]). 

 Besides these, a large number of hand- specimens from all the 

 localities have been examined. 



1 Trans. Phil. Soc. Camb. vol. i. 1822. ^ q_ j_ q^ g, yo]_ xxxviii. p. 26, 1882. 



