172 • H. HICKS OjST THE SUCCESSION OF 



been accumulated, it is evident that it must span over an enormous 

 period, and, in consequence, that it must contain within it evidences 

 of the presence of many separate and distinct faunas. I propose 

 therefore, for convenience of reference, and for the purpose of com- 

 parison with the series in other districts in which the succession 

 may not be so clear, to divide the group, by means chiefly of the 

 fossil zones, into three subgroups, under the names Lower, Middle, 

 and Upper Areilig. 



The Lower Arenig series is made up of very fine black slates and 

 shales, with a thickness of about 1000 feet, and with the beds gene- 

 rally at a very high angle. In the north end of Ramsey Island, as 

 shown in Section II. (fig. 3), they are seen resting conformably on 

 the Tremadoc group, with a nearly vertical dip, and with a strike 

 from N.N.E. to S.S.W. ; but the upper portion of the series is cut off 

 by a fault which brings them against Lingula-nags dipping in the 

 opposite direction. In the middle portion of the island they occur 

 in greater thickness ; but the succession there is considerably inter- 

 fered with by faults, and the order of the beds can only be made out 

 by the fossil evidence. 



In Section III. (fig. 4), which is taken along the north coast of White- 

 sand Bay, the chief portion of the series may be seen faulted against 

 the lower beds of the Tremadoc group, the intervening beds having 

 been cut off by an east and west fault. A little inland, however, 

 from this spot the fault has crossed the series, and also the under- 

 lying groups ; and the succession is then seen in perfectly natural 

 border. 



In this section the strike of the beds is E.N.E. and W.S.W., with 

 a vertical dip ; and this strike is maintained for about three miles, 

 or until we reach the position of Section IY. (fig. 5), when the strike 

 varies to a more nearly east and west course. A line taken along the 

 south side of the greenstone masses called Carn-lwyd, Pen-berry, and 

 Waun-y-beddau will indicate the line of junction of the Arenig 

 group with the Tremadoc group along that strike • and the upper 

 edge of the felspathic trap to the south of Cwm-wdig, the junction 

 further east. The fossils of this subgroup have been chiefly col- 

 lected in the cliffs at Whitesand Bay, and near to the fault at Road- 

 uchaf in Ramsey Island. The species discovered before the year 

 1866 were mentioned in the Report by Mr. Salter and myself 

 to the British Association in that year, in which the beds were first 

 described under the name of the Arenig group, and their relation to 

 the other groups at Whitesand Bay and Ramsey Island first pointed 

 out. 



In the summer of 1872, however, a most valuable addition was 

 made to the fauna by the discovery at Ramsey Island, through the 

 joint labours of Mr. Homfray of Portmadoc, Mr. Lightbody of 

 Ludlow, Mr. J". Hopkinson, Mr. Xirshaw of Warwick, and myself, of 

 about twenty-two species of Graptolites, which Mr. Hopkinson 

 pronounced to be for the most part new to this country, several of 

 them being entirely new species. Since then many of these species 

 have been found, by Mr, Hopkinson and myself in the cliffs of the 



