222 Rev. J. F. Blahe — The Llanheris Unconformity. 



draw a festoon on the boundary, part of which must of course be 

 actually horizontaL It was, in fact, from this mor-e horizontal 

 2)ortion of the boundary that I was judging when I wrote. But if 

 the amount of dip be in question, none of the drawings yet given of 

 the section are accurate enough. The two rocks are broken up by 

 a number of transverse joints, along which slips appear to have 

 taken place, producing a step-like series of festoons (see Fig. 6). 

 ]f these were restored, the slope would not appear so great. 



Fig. 6. — Junction of conglomerate and breccia in the tramway cutting, 

 Llyn Padarn, looking N.E. 



My critics further state that " the unconformity is quite dis- 

 proved by the finer matrix graduating from one [the breccia] into 

 the other [the conglomerate]." In a hand-specimen the line is 

 sharp, and the two matrices are differently coloured. In the field 

 the joint face of one is smooth, being along a cleavage plane ; of the 

 other, rough, where the cleavage almost ceases. But a similarity of 

 matrix is to be expected when one rock is derived from the other. 

 This is perhaps the last district where such a similarity should be 

 used as an argument, for here we have the well-known example of 

 even a crystalline felsite yielding to the succeeding conglomerate 

 a matrix which can be scarcely distinguished from the original rock. 

 When the original is clastic we may well expect the derived 

 matrix to be absolutely indistinguishable. The fallacy, too, of this 

 argument has been already demonstrated. On this ground the 

 conglomerate overlying the " granitoidite " at Twt Hill was 

 positively asserted to belong to the same series, but it was after- 

 wards acknowledged to be of a totally different age and uncon- 

 formable. 



I do not know that this section is worth all this labouring to prove 

 its teachings. I am only forced to it by the dogmatic tone adopted 

 by my opponents. I should rather gather from the conflict of 

 opinion here how very useless a single section may be to carry 

 conviction as to the structure of a country ; and in this case the 

 further evidence enables us to dispense with it altogether. 



There is, indeed, in this district a discriminating test as to the 

 truth of my reading or of that of my critics, which may be applied 

 again and again in many places and in many ways. Immediately to 

 the north-west of Green's section lies a synclinal of pale banded' 

 slates and grits, towards which, according to my opponents, the 

 conglomerate is dipping, and which they take to overlie it. That is. 



