Vol. 56.] GEOLOGY OF j^ORTIIERN ANGLESEY. 251 



(b) Post-movement Intrusions. 



The later dykes and bosses are of both acid and basic composition, 

 and not infrequently both acid and basic magmas are found to have 

 been injected into the same fissure, forming composite dykes. 



Basic Dykes. — A number of these are shown on the Geological 

 Survey map cutting through the beds of the Green Series and ex- 

 tending with a general parallelism in a south-easterly or east-south- 

 easterly direction to the boundary-thrust, where they cease. They 

 are best exposed on the coast, their inland course being usually 

 concealed by the cover of Glacial Drift and soil. They are more 

 numerous than the Survey map shows, but this is a matter of small 

 topographical importance, as they rarely exceed 5 or 6 feet in width, 

 most of them being only 2 or 3 feet across. I counted over twenty 

 of these dykes along the coast in the western part of the district 

 between the fault at Porth Newydd and Trwyn Cemlyn, and they 

 occur with about the same frequency from Cemlyn Bay to Porth 

 Wnol. In the crush-zone at Penrhyn and around the harbour at 

 Cemaes they are more closely grouped together ; from Cemaes to 

 Llanbadrig they are rare, but in the broken rocks near Llanbadrig 

 Church they become again numerous. With the exception of one 

 near Ogof Gynfor, I have detected no dykes between Llanbadrig and 

 Bull Bay. 



That the d\kes at Penrhyn are of later date than the period of 

 crushing has been pointed out by Sir Archibald Geikie, 1 and those at 

 Llanbadrig show similarly their unmistakable post-movement age. 

 The course of one of these latter is represented in fig. 5 (p. 248), from 

 which it will be observed that the dyke is fairly straight, but 

 fluctuates somewhat in width, and the flow of the magma has been 

 influenced to some extent by the joints and other structural planes 

 of the rocks into which it was injected. 



I have not attempted to trace the dykes in the neighbourhood of 

 the boundary-thrust, but, according to the Geological Survey map, 

 though they pass up to the thrust they do not extend beyond it. 

 As they are of later date than the thrusting this is remarkable, but 

 possibly their absence may be merely apparent, owing to the poorness 

 of exposures in the soft Ordovician rocks south of the Green Series. 

 But the dykes really seem, for some reason as yet unexplained, to 

 have usually avoided Ordovician beds ; and I have seen only one 

 example, between the cliff of graptolitic shale in Porth Padrig and 

 the Vicarage, where a dyke is in contact with Llandeilo rocks. 



Yet, although of later date than the last period of great movement 

 in Anglesey, these dykes yield some evidence of minor movement 

 since their injection. One of the Llanbadrig dykes is truncated by 

 a small thrust from the north (fig. 6, p. 252), but it seems possible 

 to explain it as the result of disturbance caused by the rather later 

 injection of a neighbouring fissure. A microscope-section [K.A. 124] 



1 Geol. Mag. 1896. p. 481 ; see also Quart. Joum. Geol. Soc. vol. lv (1899) 

 p. 660 (fig. 9) & p. 666? 



