472 EEV. J. r. BLAKE OIT THE 



are doubtless due to the induration of a volcanic mud, and are 

 nearly allied to the marbled slates, but of more earthy material 

 and quite different appearance. They are occasionally full of 

 angular fragments of over an inch in diameter, forming a local 

 agglomerate, and at one place they appear to be cleaved. I call 

 them pelite, and "when in lenticles lenticular pelite. Considerable 

 quantities of chlorite or sericite in small specks are developed in 

 many parts, so that they are as altered as their nature permits, far 

 more so in fact than the slaty portions of the underlying series ; but 

 they are not constant in this respect. With the exception of certain 

 peculiar developments to be noticed later, this is all we can see 

 inland to the south of Llanrhyddlad. The coast is more instructive. 



Yery little of value can be made out at Porth-y-defaid itself. It 

 is a spot of great confusion, in which the grey schists have ended in 

 contortions along a zigzag line, beyond which we find a broken 

 area of marbled slate, purple slate, hard blue grit, and epidote knobs. 

 Passing north over the coarse green ashes, we find the marbled 

 slate at the corner undisturbed. The bay is formed of softer ashy 

 rocks, and the headland beyond Trefadog is of very tough material, 

 weathering into honeycombed masses, as though, by the solution of 

 some mineral that had segregated into lumps. In this there is not 

 the vestige of a dividing line of any sort beyond the modern cracks. 

 More than a mile of sea-coast, studded with these tooth-like pro- 

 jections, and cliffs continuous with them, in which no stratification 

 can be seen, and which are yet not crystalline, serves more than any 

 place in Anglesey to impress upon the geologist the volcanic origin 

 of these tuffs. In the scarped sides of such a modern volcano as 

 that of Ischia, similar phenomena may be admirably seen, but in no 

 other circumstances can they even be imagined. In the southern 

 part of Church Bay, and also in the northern, the tuffs are very soft 

 and earthy, and quite modern in appearance, though here and there 

 compressed into a banded rock ; but in the centre from Perth Crug- 

 mor to Perth Sutan are the lenticular pelites, passing into the 

 marbled slates. In these variations we see the essentially local 

 character of the several deposits. We might of course imagine that 

 they lay in bands which we could trace inland ; but tbis is not the 

 case ; whichever way the bands may be supposed to run, we find 

 the supposition contradicted by the occurrence of the pelite. The 

 softer parts, at least, are in isolated patches, which the sea has 

 nearly consumed. I^ot only in their character, but also in their 

 distribution, these deposits are of a volcanic nature, and the dif- 

 ferent masses are dependent not so much on time as the localization 

 of successive outbursts. In one direction may be carried the finer 

 dust, producing the marbled slates ; in another the coarser, more 

 basic mud, producing the pelites ; in a third the still coarser, pro- 

 ducing the grits ; and in a fourth the ejection of large fragments 

 may produce the agglomerates. 



The country north of Llanrhyddlad is of much higher elevation ; 

 the accumulation has probably been thicker ; we are nearer the 

 centre of eruption, and the ground is cut up by pairs of trough- 



