Vol. 6S,} PKE-CAMBKIAN AND CAMBEIAN OF PEMBROKESHIRE . 381 



dykes are not of later date, for similar dykes of post-Cambrian age 

 occur in the district, and most of the basic dykes near St. David's 

 are later than the faulting which has aft'ected the Cambrian deposits 

 of that area. 



It is perhaps rash, therefore, to assign any of these rocks to the 

 pre-Cambrian without definite proof of their age ; on the other 

 hand, a dyke-phase forming the latest episode of the intrusions 

 agrees with what is observable in many other areas such as the 

 Lizard Peninsula, South Pembrokeshire, etc. 



These dykes are not often exposed, but we have seen a sufficient 

 number of them to make it probable that their importance has 

 been underestimated. 



The areas occupied by the various Dimetian types are clearly 

 indicated on the Map (PI. XL), so that little need be said con- 

 cerning their distribution — except that the quartz-porphyry covers 

 by far the largest area, and is the most persistent type. The diorite, 

 on the contrary, occupies but a small area, and only one mass has 

 been detected. 



In a later portion of this paper we have discussed the relation of 

 the Cambrian rocks to the older deposits, and assuming that we have 

 good evidence of the pre-Cambrian age of all the intrusive rock- 

 types mentioned above, it remains here to demonstrate if possible 

 the relation of these types to each other and to the Pebidian. Let 

 us first consider the granite and the quartz-porphyry. 



It is unfortunate that there are, so far as we can ascertain, 

 no natural junctions between the various masses of granite and 

 the Pebidian; but, as the granite and the quartz-porphyry are 

 without doubt merely different phases of the same rock, and locally 

 pass one into the other, it will suffice to deal with that which 

 occupies the larger area and stands in clearer relationship to the 

 Pebidian. 



Where possible, the boundaries of the quartz-porphyry-masses 

 were traced with some care; and, although there are no clear 

 junctions exposed between these masses and the older masses of 

 the pre-Cambrian, the mapping brings out the fact that their 

 outlines are far from regular and that their boundaries often cross 

 the strike of the Pebidian rocks in a manner inexplicable on the 

 supposition of any reasonable system of faulting. As an instance 

 of this, we may cite the mass of quartz-porphyry to the north of 

 Barnard's Hill, and those of Tre-rhos, Brawdy, Hayscastle, and 

 Hhindaston. 



We were fortunate enough to be able to follow the boundaries 

 of several of the masses, especially of the first-mentioned mass, 

 with considerable exactitude ; it was made quite clear to us that 

 the quartz-porphyry was intrusive into the Pebidian, and partook 

 probably of the nature of a boss rather than of that of a sill. The 

 small elliptical mass occurring in the valley between Tre-rhos and 

 JFfynnon-gron added evidence in favour of this view. 



It is certain that neither the granite nor the quartz-porphyry 

 cuts or alters in any way any member of the Cambrian System. 



