162 E. Greenly — Age of Later Anglesey Dykes. 



and olivine is of exceptional occurrence. Dykes and other intrusions 

 of the Carboniferous- Permian set elsewhere in Britain seem to be in 

 general decidedly basic, and usually can-y olivine. They might be 

 matched among the Tertiary rocks of the Western Isles, which have 

 a very considerable range of composition, basic and sub-basic; but 

 the Tertiary dykes of the North of England are, as Teall has pointed 

 out, mainly of andesitic nature." 



On p. 215 of "British Petrography," Mr. Teall remarks on a rock 

 from Holyhead Island as " a wonderfully fresh ophitic olivine 

 dolerite. In its composition and state of preservation it differs 

 so markedly from the ' gi-eenstones ' and approaches so closely to 

 many of the Carboniferous and Tertiary dolerites that one is inclined 

 to regard it as a rock of much later date." 



The Holland Arms dyke presents two opposite extremes with 

 regard to state of preservation. Much of the rock is decomposed to 

 a sand,^ and yet in the cores of the spheroids even such a mineral as 



Fig. 1. — Group of dykes in MjTiydd Llwj-diarth. 

 olivine remains intact and unserpentinized to a degree almost 

 unknown in Pre-Tertiary rocks. Mr. Barrow tells me that this 

 condition is characteristic of the Cleveland dyke when traversing 

 certain beds in East Yorkshire. The less basic dolerites are also in 

 general very well preserved. 



In their strike and general behaviour in the field these dykes closely 

 resemble those of the West of Scotland ; and certain parts of the 

 maps can hardly fail to at least suggest to the mind that they belong 

 to the same age and series of intrusions, the type of volcanic action 

 being identical. (Eig. 1.) The average strike is about N.W.-S.E., 

 a large number ranging W.N.W.-E.S.E., and a few in a more N.-S. 



1 Denudation along this soft friable material probably initiated the transverse 

 ravine on the south side of the Holyhead Road at Holland Arms. 



