312 Prof. Sedgwick on the May Hill Sandstone, 



with those previously observed by Mr. Salter, were, however, 

 sufficient for our purpose, and enabled us to draw the following 

 conclusions : — 



(1.) There can now be very little doubt about the geological 

 place of the Hollybush Olenus shales. They are where I had 

 provisionally placed them in my last paper ; adopting without 

 reserve the published views of Professor Phillips, who first dis- 

 covered and described them. 



Near Shineton, there is nothing in the sections to indicate 

 the protrusion of any very ancient rock ; nor any igneous rock 

 (like the Malvern syenite) to disturb the relations of the neigh- 

 bouring strata. Hence it appears certain that the Shineton 

 beds (although inferior to the above-mentioned beds of Harnage 

 Grange) must be arranged with the shales which form the base 

 of the Horderley or Caradoc terrace ; in which case they must 

 be subordinate to the Bala group. 



The note of interrogation after the word Olenus in the pre- 

 vious list may perhaps show that Prof. M'Coy is not certain as 

 to the genus; but, whatever it may be hereafter called, he is certain 

 that it is identical with the Olenus of the Hollybush shales. 



(2.) The (supposed) typical section through Caer Caradoc, 

 Wenlock Edge, &c., is not a truly continuous, but a broken 

 section. The true Caradoc sandstone and Caradoc shale do not 

 graduate into the beds which immediately surmount them ; and 

 the conglomerates, grits, and Pentamerus limestone, &c. (which 

 discordantly overlie the true Caradoc sandstone) must here, as 

 at the south end of the Malvern Hills, be cut off from the Cara- 

 doc terrace and arranged with the Wenlock group. 



(3.) From the above facts, and from all I have seen or learnt 

 of the older palaeozoic rocks of North and South Wales and the 

 neighbouring English counties, I think I may conclude, that in 

 these great physical regions there is not so much as one con- 

 tinuous unbroken section through which we can ascend, in the 

 way of passage, from the Cambrian to the overlying Silurian 

 groups. There is a physical break between them exactly on the 

 horizon of the May Hill sandstone; and in very exact co-ordi- 

 nation with that break (sometimes distinctly marked by a dis- 

 cordancy in the position of the beds), there is a great change in 

 the fossil species. If this be true, and I know nothing to oppose 

 to it, we have at length found the true physical and palseonto- 

 logical base of a " Silurian System.'^ 



The accompanying section (fig. 4) represents what is, I 

 believe, the true sequence of the deposits in the Caradoc section. 

 Between the Longmynd slate and the overlying slate and sand- 

 stone, there is great interval, the filling up of which requires 

 the interpolation of, at the least, 20,000 feet of strata, from the 



