534 KEY. J. r. BLAKE Olf THE 



is the statement that the Cambrian rocks have been faulted down, and 

 are not in the slightest degree altered at the junction. This supposed 

 fault would have to be a circular or ellix^tical one, and we are not told 

 whether the junction rocks have been examined by the microscope, 

 or whether they are brecciated. Of Bwlch 3tlawr, Pen-llechog, and 

 Yr Eifl, Dr. Hicks writes, " the rocks are in some respects unlike the 

 rocks already described, but yet clearly of that type and age," and 

 '^ Upper Cambrian rocks are faulted against these masses and are in 

 no case altered, except near dykes.'' This is the theory, presented 

 without stratigraphical proof. The object appears to be to show that 

 the rocks in question are lava-flows and eruptive products, and not 

 intrusive masses. The same arguments are used respecting the 

 masses at Nevin, Boduan, and Pwllheli. Kow with regard to several 

 of these areas, especially Tr Eifl and Boduan, it may be at once 

 admitted that they are probably eruptive and not intrusive. The 

 masses coloured red on the map are complex, and though some parts 

 are coarsely crystalline, others are more felsitic and banded, and may 

 well be contemporaneous eruptions. But this does not show at what 

 time they were poured out, and in fact the mass at Boduan is suc- 

 ceeded by a slate containing its pebbles, obtained by contemporaneous 

 erosion. Their age we must learn by their stratigraphy. jS'ow as to 

 any supposed faults, either the Survey mapping is all wrong, or else 

 the course of the faults would be most remarkable, curving round 

 corners and running into tongues. But I have satisfied myself that 

 within half a mile on either side of the road from Xevin to Clynnog 

 Pawr, which includes some of the most remarkable of the supposed 

 curved faults, the mapping is quite right. Moreover, in Carn Bod- 

 uan on the west side, in the Mynydd Nevin near Pistill, on the 

 north side of Tr Eifl, overlooking the highroad, all of which 

 I have examined close at hand, and looked at from a distance; 

 and on Pen-llechog and Tstum Llech, so far as their deep stream- 

 worn gorges reveal their structure to a distant observer on the 

 road — in all these places the fault would have to be horizontal, 

 for in all of them the igneous rocks are seen overlying the slates. 

 In other words the stratigraphy proves conclusively that they are 

 of later date than the underlying portion of the Ordoviciau, and 

 the argument from their similarity to known Pre-Cambrian rocks, 

 an argument which on both points is untenable, entirely falls to 

 the ground. 



HowTH Hill and Beat Head. 



The rocks of these localities are so well known, and so admirably 

 described in the Memoirs of the Irish Geological Survey and else- 

 where, that they do not need any further description, except from 

 the point of view of their relation to the rocks of Anglesey, to which 

 they are nearest on the west. That they have been thought to 

 have some relation to these is obvious, from the fact that those who 

 look upon the Anglesey rocks as Cambrian have placed these Irish 

 rocks in the same system ; while those who have found Pre-Cam- 

 brian rocks in Wales have looked upon the Howth rocks at least 



