Vol. 55.] GEOLO&T OF NORTHERN ANGLESEY. 661 



lie in lenticular patches among the shales. Pieces of these grit- 

 beds form * pebbles,' often with slickensided surfaces ; and angular 

 fragments of the shale, pebbles of quartz and grit, and these 

 * pseudo-pebbles ' are all felted together into a solid rock. Most of 

 the pebbly shales seem to have been, on the whole, formed in the 

 ordinary way by the denudation of older rocks. To the south the 

 slates return to their ordinary character, but bands of grit lie more 

 or less isolated, showing that there is still disruption, though in a 

 less severe form. 



This locality (i) introduces us to a crush-conglomerate in 

 Ordoviciau beds, and hence fixes the age of the movement as post- 

 Ordovician ; (ii) shows us a deposit consisting partly of pebbles of 

 earlier date, formed by denudation, and partly of pseudo-pebbles 

 derived from neighbouring beds of contemporaneous age by 

 crushing. Whence we may learn that the test of included frag- 

 ments to determine the relative age of beds, largely used in un- 

 ravelling Anglesey geology, must be verj'- cautiously applied. 



(iii) The Cemaes District. — The largest and most typical 

 development of the crush-breccias is exposed along the coast in the 

 neighbourhood of Cemaes at Porth Wnol, on Wylfa Head, and along 

 the shores of Cemaes Bay. Immediately south of this remarkable 

 zone lie the green- an d-purple slaty beds which occupy the greater 

 part of the jSTorthern District, and compose the crush-zone at 

 Trwyn Pen-careg. As we strike across the green-and-purple beds 

 northward, say from Porth y Pistyll towards Wylfa Head, they 

 are seen to dip a little east of north at a low angle (10° to 20°), 

 increasing to 40° at Porth y Gwartheg. Patches of brecciated 

 rock, agreeing in character with the Trwyn Pen-careg breccia, are 

 visible here and there. Contortions set in, whose axes agree in 

 direction of dip with the bedding, and the bedding becomes 

 irregular, so that thin, harder, and flaggy bands in them appear to 

 die out if followed for a few inches or feet along the strike. 

 On reaching Porth Wnol these beds still form the main mass of the 

 exposures ; but in them lie pieces of the gritty beds of the series 

 and great lumps of greenish glassy quartzite that weathers with a 

 white crust ; a still larger mass near by forms one of the * quartz- 

 knobs ' of Prof. Blake. 



On this headland, and along the shores between Porth yr Ogof 

 and Porth y Wylfa, the slaty rocks are thoroughly broken up, and 

 contain many inclusions of grit and quartzite. A few limestone- 

 masses are folded in among the strata. One of these, of which 

 about 30 feet in length is seen, is of such irregular shape that, had 

 it been, as has been suggested, ejected from a volcano, it would have 

 broken into several pieces in falling. 



I may mention here that limestone-' pebbles ' in these rocks 

 are not common, and such few as are seen generally occur in the 

 neighbourhood of these limestone-masses. 



The great majority of the inclusions are clearly identical with the 

 grits that occur in thin bands to the south, and in thicker beds 



