476 EEY. J. F. ELAKE ON THE 



fragments into pebbles, and has finally filled up the cavity by the 

 deposition of its quartz. In other words, they are the bases of Pre- 

 Cambrian geysers, which may, or may not, have succeeded in 

 reaching the surface and erupting. In the western district these 

 are seen at Yr-ogo-r-arian and south of Porih-yr-hwch in the 

 area round Pen-bryn-yr-Eglwys, on the east side of the road near 

 Llanfaethlu, and near the river by Clwch-hir, north of Lland- 

 deusant ; but much more characteristic and important examples 

 are found in other districts. They are always associated with the 

 rocks which we have other reasons for concluding are of volcanic 

 origin ; and this is one reason for looking to the phenomena of such 

 regions for their cause. 



3. Sporadic Limestones. — These are elongated bands of crystalline 

 limestone filling lenticular spaces in the pelites ; as they are quite 

 isolated and irregular and have no constant strike, we cannot con- 

 sider them as beds which are now placed on end, but must look on 

 them as products of infiltration or segregation. They are composed 

 of a mosaic of small crystals, in which there may be larger crystals 

 with abundant rhombohedral cleavage. In other districts and 

 higher in the series we meet with limestones of more bedded 

 character, but these may have been produced on the surface by 

 the same waters which caused the present ones below. These 

 sporadic limestones are seen at Llanfaethlu, where nests of umber 

 occur in the rock ; also at the telegraph- station near the same 

 place, and on the hill-slopes to the west of Ogo Lowry. They must 

 not be confounded with the calcific bands in the older rocks, nor 

 with the remarkable inburst at Perth Delise already described. 



The South-Stack Sehies. — On the south-western side of the main 

 fault that divides Holyhead Island into two parts, the rocks are of 

 a different character from any we have yet seen, and ought to be more 

 sharply marked off from the Holyhead group than they have hitherto 

 been. I therefore designate them the South-Stack Series, as they 

 are well seen at the South-Stack Lighthouse. In certain parts, no 

 doubt, as at the lighthouse itself, their splendid contortions and 

 their rugged aspect defying the stormy sea, give them an appearance 

 of great antiquity, and it is on this account that Sir A. Eamsay con- 

 siders the fault to have a downthrow on the N.E., by which the 

 rocks of Holyhead mountain are made the younger. Much colour 

 is lent to this view by the fact that throughout the long succession 

 which we can elsewhere trace in the island, nothing like them is to 

 be found. Nevertheless, the entire absence of metamorphism in con- 

 siderable parts, and their resemblance to the bedded rocks of Bray, 

 and even to some portions of the Cambrian of Wales (a resemblance 

 which would have no weight with Sir A. Eamsay, who considered 

 all to be Cambrian), inclined me from the very first to the opposite 

 ■ view, and I believe I can now prot;^ that they are younger. 



The argument from the degree of metamorphism amounts to very 

 little. AVe have already seen that on the north-eastern side of the 

 fault the chloritic schists, which are thoroughly foliated, are one 



