470 EEV. J. r. BLATTF, ON THE 



But it is to the sea-coast that we must go to see the true character of 

 the series. Leaving the turnpike road between Yalley and Holyhead, 

 we hardl}" get foot on the rocks which the sea has exposed, before 

 we receive a somewhat astonishing lesson. At first we walk on the 

 ordinary chloritic schist, but soon this begins to have a mixed 

 appearance, being more slaty and touched with purple ; and then in 

 little space the same bed has changed into a regular purple slate. 

 Sometimes the change may be traced vertically, and sometimes 

 horizontally ; but by one way or the other, before the mouth of the 

 Alaw is reached, the whole shore is a mass of purple slate. It 

 ought rather perhaps to be called shale, since there is no definite 

 cleavage, though the rock is divided by lines of pressure parallel to 

 the bedding. In structure it consists of very fine dust, and, with 

 the exception of fine flakes of sericite here and there, is in no way 

 altered. Here, then, in the very midst of the chloritic schists, and 

 forming part and parcel of them, we have a slate that is entirely 

 comparable to the Cambrian. The colour, however, of the latter 

 is usually a bluish purple, while this, like the rocks of the Long- 

 mynd, is of a reddish purple. We must certainly excuse any author 

 who had observed these facts for being led to consider the whole 

 of these most ancient rocks to be only altered Cambrian. 



On crossing the Alaw we come again to chloritic schists which 

 are nearly horizontal, and are associated seawards vrith massive 

 grits and epidotic rocks. In these begin to be seen some scattered 

 fragments of quartz and felspar, which are not of the same order of 

 magnitude as the remainder of the rock, and, being quite angular, 

 suggest a volcanic origin. The grits especially are full of these. 

 Amongst the fragments thas contained in a rock essentially of the 

 lower part of the series are some which may be described as quartz 

 with many chlorite inclusions, but which are not to be confounded 

 with chloritic schists. 



Following the rocks containing these, we find, as we go north, a 

 new type consisting apparently of the finest dust in lenticular patches, 

 coloured brown and green, and showing no stratification beyond 

 these lenticles. Under the microscope these are seen to consist of 

 an excessively fine mosaic of crystalline particles with very little 

 that is opaque, separated by segregation lines of a coarser mosaic of 

 quartz. As the total result is a slaty rock, I propose to refer to 

 this, which is of wide distribution, as a marhJed slate. I conceive 

 that it must be produced by the accumulation of volcanic dust. 

 Further north at Peniel, we come to grey ashy beds dipping to the 

 S.E., and these are followed on the north side of Porth-penryn-mawr 

 by laminated schists, changing, as we have seen them do before, 

 into purple slates, the boundaries of the latter in one instance being 

 a pair of joints. "We thus have a synclinal with the volcanic dust 

 and ashy beds lying on the chloritic schists and their unaltered 

 representatives. After a sharp anticlinal turning the beds over to the 

 N.W., we find a disturbance and a fault, and then chloritic schists 

 come on again horizontally, and are only disturbed again at Perth 

 Delise, where there is an intrusion of epidote-rock, and a curious- 



