PBE-CAMBEIAN EOCKS OP ST. DAVH>'s. 289 



fragments continue to appear in the conformable sedimentary deposits 

 that overlie ancient volcanic rocks. The relation of the conglomerate 

 to the rocks underneath is nowhere better seen than in the ex- 

 cellent exposure of the band on the west side of the diabase crag at 

 Bhosson. The conglomerate there rests almost immediately on the 

 igneous rock, yet 95 per cent, of its pebbles are of quartzite and 

 quartz, only 4 per cent, being diabase, and these derived from some 

 different sheet than that immediately below *. 



As a part of the arguments from included fragments, reference 

 may be made to Dr. Hicks's statement that the volcanic group must 

 have been much in its present lithological condition before a grain 

 of the Cambrian rocks was deposited. The rocks of that group 

 were called " Altered Cambrian " on the Survey Hap, doubtless 

 because it was recognized that, whatever might have been their 

 original condition, they had undergone considerable alteration. 

 This altered condition is one of the first features of the rocks to 

 arrest attention. It is not universal, indeed, and is by no means 

 uniform, but in certain bands is so marked that the strata have 

 passed into true silky schists with well-developed foliation. This 

 alteration will be more fully dwelt upon in Part IT. 



Now, if it could be shown that the metamorphism stops short 

 below the conglomerate, not affecting the beds above that horizon, I 

 admit that the fact might be plausibly used to sustain an attempt 

 to draw a strong line between the conglomerate and the rocks 

 below it. But no such restriction exists. The shales far above 

 the conglomerate have undergone as complete an alteration, and 

 present examples of schists as perfectly foliated as those in the 

 volcanic group beneath. Pine silvery and silky schists are interca- 

 lated among grey and purple sandstones that overlie the quartz 

 conglomerate in Porth-Seli, at the south end of Whitesand Bay, and 

 at other localities (see Part II. p. 311). 



The facts now stated would be sufficient to disprove the assertion 

 that there is any break in -the succession of deposits at the base of 

 the conglomerate. But to complete the argument one further 

 destructive fact remains to be noticed, viz. that the volcanic beds 

 do not cease below the conglomerate, but are interstratified with it 

 and appear above it. In proof of this statement I may refer to the 

 sections on both sides of the fold where this perfect passage can be 

 seen. On the shore of Ramsey Sound, at the headland of Castell, 

 the red shales and sandstones with Lingalella primceva, which lie 

 not far above the conglomerate, are banded with thin seams of 

 sandy tuff, some of the shales being also full of diffused tuffaceous 

 material, as if from slight discharges of fine volcanic dust during the 

 last stages of eruption in the district. 



Again, on the east side of the isocline in the valley of the river 

 Allan, the interesting section shown in fig. 8 may be seen at the 

 upper end of the first field north from the bridge near Lower Mill 



* This locality is referred to by Dr. Hicks (Q. J. G. S. vol. xxsiii. p. 237, 

 1877) as showing the unconformability of the conglomerate on the rocks below. 

 I cannot conceive how such an assertion can have been made. 



