468 EEY. J. r. BLAKE ON THE 



and fragments wHch are still found in them. The welding together 

 of alternating ingredients in the process of crystallization has pro- 

 duced a very tough material, and subjected as it has been to enormous 

 pressure, it refuses to cleave ; it will not be faulted or broken, or 

 bend bodily into massive folds ; all it will do is to crumple, or, as 

 Prof. Hughes calls it, to " gnarl." When seen, as it often is, on 

 the surface of a rugged boss of rock, it gives the boss a damascened 

 appearance by the fine and intricate patterns of its various lines. 

 When it has been less compressed, as in the road-cutting near Porth- 

 yr-ogof, we can follow the lines on their crinkly path and obtain 

 the general direction of the dip. When the pressure has been mostly 

 perpendicular to the lamiufe, the rock becomes massive and slaty, 

 and the materials are less altered, though some foliation has taken 

 place. Thus the gnarling is a subsequent process. As we pass 

 eastwards from Holyhead several more massive zones are encountered, 

 containing more quartzose grains ; but with this slight variation, the 

 remainder of the island, as exposed inland on the ]^,E. side of the 

 main fault, shows only these schists belonging to a stratified series. 

 It is to be noted, however, that our general means of observation, 

 are limited to the bosses that protrude through the fields, and these, 

 from their very mode of occurrence, are probably the more indurated 

 and thoroughly crystalline portioiis of the rock, while any softer or 

 less altered part would be hidden beneath the soil. But the sea- 

 shore knows no such rule, and accordinglj- between the railway 

 and Gorsedd-y-Penryn on the island shore the more crystalline 

 masses are interbedded with more slaty and purple varieties, and 

 there is a band of browner rock containing angular fragments of 

 quartz and felspar of comparatively large size. 



Before leaving Holyhead Island, we may notice a remarkable out- 

 lier of coarse conglomerate which lies on the west side of the great 

 quarries, and forms the foundation for some buildings. The frag- 

 ments are so large and the deposit so local that one thinks at first 

 of an artificial concrete ; but the matrix, on examination, is seen to 

 be inimitable by man. The fragments are not those of the neigh- 

 bouring rocks, but resemble the higher beds of the Pre-Cambrian to 

 be presently described. This seems to die out suddenly, as by a fault, 

 towards the sea ; but towards the rifle-pits it tails off gradually, and 

 contains more of the local quartzite. We can scarcely fail to re- 

 cognize in this deposit a basal Ordovician beach-breccia, the in- 

 terest of which lies in the proof it affords that the Pre-Cambrian 

 rocks were worn down in this district to near their base during the 

 continuance of the Cambrian era. 



The aeea Noeth of the Holyhead Sieaits. — At Porth-y-defaid, 

 about four miles north of YaUe3\ Dr. Callaway draws an important 

 fault, running inland with an E.T^.E. trend, which, he says, separates 

 entirely a "slaty" series on the north from a " gneissose " series on 

 the south. At this point also on the coast, the legend on the Survey 

 Map is changed, the country on the south being characterized as 

 " green and purple schists, often micaceous," while that on the 



