514 EEV. J. p. BLAKE ON THE 



Boundary of the District. — It is stated by Sir A. Ramsay that its 

 southern boundary is a remarkable curved fault, and this, Dr. Calla- 

 way, at considerable trouble, has confirmed. Nevertheless, according 

 to the views of Prof. Hughes, this fault, if it occurs, can be of little 

 stratigraphical importance, and Dr. Roberts has attempted to prove, 

 by the evidence of two isolated inland localities, that it cannot exist. 

 It is not, however, upon evidence of this kind that such a fault can 

 be satisfactorily proved or disproved ; we must have recourse to 

 the general stratigraphy. Now in this Northern District a very 

 definite sequence of rocks may be demonstrated. Being less meta- 

 morphosed, they are also, with few exceptions, less disturbed ; and 

 we can follow the strike of each with considerable accuracy. Now, 

 whatever part of the series we may be on, from the lowest to the 

 highest, and therefore, whatever class of rock is on the northern side 

 of the fault, as soon as we overstep that boundary, we are imme- 

 diately landed in black shales, which have a pretty uniform character 

 throughout. This leaves no alternative but a fault or an uncon- 

 formity ; if it were an unconformity, the upper group would run 

 parallel to the boundary, which it does not : and we might expect 

 to find a conglomerate somewhere at the line of junction ; but we 

 never do. On the contrary, as Dr. Callaway has shown, the con- 

 glomerates are in the Ordovician, and they contain fragments of 

 Cemmaes-limestone and of other rocks of the northern series. 

 These observations and the fact that where the junction is actually 

 seen, either at Porth-yr-ysgraff on the west or at Porth-yr-corwg on 

 the east, it is admitted by all to be faulted, seem to be satisfactory 

 proof that there is a bounding-fault throughout. Still no evidence 

 beyond the fragments in the conglomerate is produced as to the 

 direction of the throw, and the Northern District, so far as stra- 

 tigraphy goes, might have been let down and not pushed up. 

 In fact, at both ends where the fault is seen it hades to the north. 

 We should thus have to consider it reversed, if the northern 

 series be the older. This, of course, is no difficulty, but it shows 

 that the age of the series cannot be entirely determined by the 

 stratigraphy, but must be dependent on the character of the rocks 

 themselves. 



The SEDnrENTAET Series. — This portion of the district, though 

 subject to local contortions, has on the whole a pretty uniform dip 

 towards the north, though variable in amount. Towards the west it 

 is low and inclines somewhat to the east ; in the centre some dips 

 may be measured at 60° ; and in the east, where any can be made 

 out, they approach to 90°. We may therefore expect to find the 

 lowest beds of the series at the southernmost bend of the fault, that 

 is, at Llanflewin. Here the northern and western districts approach 

 within a mile of each other, and the rocks in the former are thoroughly 

 foliated and gnarled chloritic schists, more like the rocks of Holy- 

 head than those of the neighbouring western district, yet really of 

 the same type ; and if any value is to be attached to lithological 

 characters, either on the large or small scale, there can be no reason 



