674 ME. C. A. MATLEY ON THE [Aug. 1 899, 



been brought np against the Llandeilo conglomerates by an over- 

 thrust-fault. The limestone is therefore older, and there are occa- 

 sional fragments probably derived from it in these neighbouring 

 conglomerates. Also the conglomerates contain abundance of 

 quartzite-pebbles, and lie on the quartzite, so that the quartzite is 

 older than the conglomerate. 



The various lithological types of Northern Anglesey — the purple 

 and pale boulder-conglomerates, the sporadic outcrops of limestone, 

 the shales and flaggy beds of varied colours, the serpentines and other 

 intrusions — have a remarkable resemblance, as Prof. Lapworth 

 pointed out to me, to the general facies of the Girvan and Bal- 

 lantrae rocks of Southern Scotland. Now the Lower Girvan rocks 

 do correspond in age with the recognized Ordovician of Northern 

 Anglesey, and, like the latter, their basal bed is a purple, red, or 

 white conglomerate which lies on the Ballantrae rocks. The latter, 

 once suggested to be pre-Cambrian, have proved to be no older than 

 the Arenig. It therefore deserves consideration whether the two 

 areas do not correspond in more respects than in lithological simi- 

 larity, and whether we may not recognize in the Green Beds or in 

 the Northern Complex, or in both sets of strata of Northern Anglesey, 

 a series of rocks not very much older than the Llandeilo. We find, 

 for instance, that black shales typical of the undoubted Ordovician 

 are found among the pre-Llandeilo limestones, and that the purple 

 colour which characterizes certain zones in the Green Series is not 

 wanting in the Llandeilo conglomerates. On the other hand, the 

 included fragments in these conglomerates prove that the Green Series 

 and the quartzites had already undergone much alteration; moreover, 

 the green rocks are correlated by geologists with the pre-Cambrian 

 of other parts of Anglesey, and there is also the general argument 

 (p. 643) that Northern Anglesey remained above water throughout 

 Cambrian and Arenig times. Swayed by these conflicting con- 

 siderations, we are not in a position to assert unhesitatingly whether 

 the Northern Anglesey rocks below the Llandeilo are of Arenig, 

 Cambrian, or pre-Cambrian age. Though evidences appear to favour 

 the opinion that these rocks, and more especially the Green Series, 

 are very much older than the Ordovician conglomeratic base, it seems 

 better to adopt a cautious attitude, and until the discovery of fossils, 

 or pending the results of further work, to use, in speaking of their 

 age, the general term pre-Llandeilo. 



YI. SUMMAET. 



The principal conclusions arrived at in the foregoing pages may be 

 summed up in the following statements : — 



(1) The barren green strata which form the floor of the greater 

 part of Northern Anglesey are older than the Ordovician rocks to the 

 south, and owe their apparently overlying position to a great over- 

 thrust which sweeps in a curve from sea to sea. 



(2) In the northernmost part of the island is a complex of rocks 

 of varied lithological character. Among them occurs a group of 



