H. HICKS ON XHR PEE-CAMBBTAN EOCKS OF ST. DAVId's. 235 



One of these dykes may be examined just above the last limestone- 

 band in section III., and another in the Porthclais valley on the 

 east side near the lime-kilns. 



Pebidian. 



These rocks everywhere rest unconformably on the Dirnetian, the 

 beds being- usually at a very high angle, almost vertical and sometimes 

 even slightly inverted. As may be seen by the map (PL X.), they ex- 

 tend all along both sides of the ridge, with the exception of a small 

 area at the north-east end and where they are cut off towards the 

 south end by faults. The strike is for the most part from W.S.W. 

 to E.N.E., or parallel with the ridge. They vary considerably in 

 thickness, showing at some places less than 500 feet, and at others 

 over 2000 feet, the thickness being chiefly regulated by the amount 

 of the overlap b}* the Cambrian conglomerates, which have a strike 

 nearly in a line with them, but dip usually at a lower angle. These 

 rocks, like the Dimetian, are always greatly metamorphosed, and 

 consist" chiefly of indurated shales, often porcellanitic in character. 

 In the more compact masses the stratification is generally indicated 

 by narrow dark lines, which give the rock a rather pretty banded 

 appearance. The beds are also frequently intersected by closely 

 approximated joints. 



The lower beds of the series, or those resting immediately on the 

 Dimetian axis, are hard compact conglomerates, in which, however, 

 from the great change which they have undergone, the distinctive 

 outline of the pebbles is for the most part lost. The conglomerate 

 seems to be chiefly composed of masses of quartz and altered shale, 

 or such masses as might have been derived from the underlying 

 rocks, afterwards very closely cemented together. The best-exposed 

 sections are at Nun's Well, on the coast south of St. David's, to the 

 east of the fault, and on both sides of the Caerbuddy valley to the 

 east of St. David's. They may also be seen in the St. David's 

 valley to the north and south of the Cathedral ; but here super- 

 ficially they differ somewhat in their appearance from those in the 

 sections to the south of the ridge, being less compact, and decom- 

 posing more readily. The more solid portions, however, when ex- 

 amined, indicate a rock of the same texture as that in the other 



first sight from some of the extremely altered schists ; but when seen, as they 

 sometimes are, associated with aud penetrating the schists, the columnar struc- 

 ture, always very marked, at once points them out. They were intruded into 

 these rocks at a very early period ; hence they have also suffered from the great 

 changes to which the rocks have been subjected. This alone might account for 

 some of the appearances, and for the similarity in character. Prof. Judd, F.Gr.S., 

 has kindly examined two specimens of this rock for me microscopically ; and he 

 reports as follows: — " The base and enclosed minerals are both pseudomorphous. 

 The rock now consis'ts of zeolitic minerals, calc-spar, and 'viridite;' but the 

 forms of the original felspar crystals can clearly be traced, and there is very little 

 doubt that the rock is a greatly altered ' dolerite.' It woidd be called by con- 

 tinental writers a ' melaphyre.' " 



