Yol. 68.] PEE-CAMBRIAN AND CAMBRIAN OF PEMBEOKESHIRE. 395 



exposures alone were examined, and the map completed on this 

 basis without reference to the debris. 



It was demonstrated over and over again that when, in any part 

 of the district which was being surveyed by means of debris- 

 indications, exposures of the solid rock were encountered so that 

 the nature and inclination of the rocks underlying the subsoil 

 could be examined directly, such exposures rarely necessitated any 

 change in the position of the lines drawn up to that point. The 

 solid exposures merely confirmed the position of lines which had 

 been drawn by reference to debris alone or, in some cases, aided by 

 such slight features as exist in that part of the country. These 

 remarks appear to be necessary, in order to explain how a map 

 showing so many subdivisions of rocks and such complicated 

 structures can be constructed in a country which is singularly 

 devoid of natural sections. 



Only one clean exposure showing the base of the Cambrian 

 actually resting on older rocks occurs in the district, namely, in 

 the lane leading to Stone Hall Mill, where the conglomeratic 

 quartzite rests upon highly -cleaved or schistose rhyolitic ashes, 

 which probably belong to a high part of the Pebidian. 



In Brawdy farmyard, however, the line of contact of the Cam- 

 brian pebbly grit with the Brawdy granite can be located with 

 precision. 



The mapping brings out the fact that the base of the Cambrian 

 adjoins widely different rocks in different localities. In most cases 

 the junction is presumably normal, for its trend is usually not 

 parallel to the main lines of faulting. If we suppose the base of 

 the Cambrian in the district between St. Lawrence and Brimaston 

 to be faulted against the older rocks, the faults would have to 

 pursue an extraordinarily undulating course in a direction contrary 

 to most of the faults of the district ; and, moreover, would have to 

 leave the Cambrian rocks unaffected, while producing considerable 

 dislocation in those adjoining. It seems more natural to assume, 

 therefore, that in this district, at any rate, the junction of the 

 Lower Cambrian with the underlying rocks is one of normal 

 superposition. 



!N'ear Stone Hall, as has been stated, the basal Cambrian rests 

 upon the Pebidian, and to the east of Newton East can be seen 

 in close proximity to similar rocks. North of Brimaston the lowest 

 Cambrian adjoins a coarse quartz-porphyry, while in two separate 

 outcrops, east of Brimaston, it stands in the same relation to the 

 Brimaston granite. 



South-east of Tre-rhos the rock which is in contact with the 

 basal Cambrian is a coarse quartz-porphyry ; while a little farther 

 south-west this rock is replaced by striped pink-and-white rhyolitic 

 tuffs, which belong to the upper part of the Pont-yr-hafod Group of 

 the Pebidian. 



On Hhindaston Mountain, and to the north of Pen-y-cwm, 

 Pebidian tuffs adjoin the Cambrian conglomerate ; but, in the long 

 stretch from Knaveston to Brawdy, Dimetian granitic rocks are 



