The Rev. Prof. Blake — Base of the Sedimentary Series. 361 



even down to the lowest. Now it is only this upper series that has 

 been shown by Dr. Callaway to contain so many rhyolite pebbles. 

 This alone, then, is necessarily younger than the Uriconian. At the 

 same time, the title " Longmyndian " ceases to be applicable to the 

 whole ; and if the upper part be Cambrian, there seems no reason to 

 disturb the existing terminology, which makes the " Longmynd 

 group" part of that system. But the Cambrian age of even this 

 upper part requires to be proved, if it be cut off by a fault and 

 bent into a synclinal on the west, as described by Dr. Callaway. 

 This point was, therefore, next examined. I could find no con- 

 tinuous fault, but only a local one near Lyds Hole, in the neighbour- 

 hood of which is the onl} r reversed dip in the district ; elsewhere it 

 is steadily towards the west, and everywhere the upper grits are 

 succeeded on the dip by pale shales, continuing up to the conformable 

 Stiper stones. Thus the series is conformable and continuous from 

 the Stiper stones downwards to the lowest gi'its, and hence we are 

 thoroughly justified in calling the latter Cambrian. I had, however, 

 to satisfy myself of the nature of the small exposures of rock which 

 had been referred to the Archaean by Dr. Callaway. They were all 

 examined with care, and evidence obtained, and elsewhere recorded, 

 that they were none of them really Archaean, or had anything to do 

 with Precambrian rocks. 



The division of the Longmynd by an unconformity into an upper 

 or " Cambrian " seines, and lower, or " Upper Monian " series, has 

 another, and unexpected result : it leaves the relative ages of the 

 Upper Monian and Uriconian undetermined, and this had next to be 

 investigated. If the two were really separated by a fault, it would 

 be hopeless ; but I found that though a fault does run in the valley 

 between Caer Caradoc and Church Stretton, it does not affect the re- 

 lation sought for, and the two groups do actually come side by side, 

 and there is a fourfold evidence that the Uriconian is the younger. 

 1. They come together along a very curved and crooked line, not to 

 be accounted for by faults. 2. The Monian beds are here very fine 

 slates, and not in any way derived from the underlying beds, as they 

 should be if younger. 3. They are much altered near the line of 

 junction. 4. Portions of them are caught up amongst the volcanic 

 rocks. Although the last two statements are the best interpretations 

 of phenomena rather than indisputable facts, all four reasons together 

 can leave little doubt that the Uriconian are the younger rocks. In 

 saying this I am changing former '■ views" on account of actual 

 evidence, and the change is far-reaching. The notion that the 

 Uriconian corresponds with any part of the rocks of Anglesey 

 (except the Beaumaris rocks) must be given up, and they must be 

 compared with the rocks of Bangor, as has been done by Prof. 

 Bonney. Lying as they are thus shown to do between Monian and 

 Cambrian, their classification is a matter of " view " only. My own 

 is that they belong to the interval between the two systems, and it 

 is a matter of indifference with which they are bracketed. The 

 evidence from other districts is conflicting, even if we are justified 

 in assuming the correctness of our correlations. The Pebidians at 

 St. Davids, like the Uriconian (their equivalent), are sharply marked 



