SEDGWICK ON THE MAY HILL SANDSTONE, 229 



that a large series of beds must be interpolated to make this Ceiriog 

 section complete*. 



Though the representatives of the May Hill grits have disappeared 

 from the Horderley sections, yet farther towards the north (for 

 example, along the north-western part of the Berwyn chain, and in the 

 long range north of the Holyhead road, which is now coloured as 

 Caradoc in the Government Map), these grits break out in great force 

 in their proper place, and with their characteristic fossils. I adhere, 

 now, to the opinion adopted on the spot (in 1842 and 1843) by Mr. 

 Salter and myself, and believe that the sandstones and conglomerates, 

 which on the line of the Holyhead road form the base of the Denbigh 

 flags, are Upper or true Silurian, and the equivalents of the May Hill 

 sandstone ; and that the same rocks are not the equivalents of the 

 sandstones of Horderley and Caer Caradoc, as they are represented 

 (under one gamboge colour) in the Map of the Survey ; the true 

 Caradoc sandstone being but an example of an upper shelly sand- 

 stone near the top of the great Bala group, and therefore a true 

 Cambrian rock. I believe, therefore, that the opinion formed by 

 Mr. Salter and myself in 1842 and 1843 was right ; and that, in this 

 part of the series, the colours on the Map of the Survey require some 

 modification. Without, however, dwelling on phsenomena which I had 

 no opportunity of revisiting during the past summer, I think I have 

 shown sufficient positive evidence for some considerable corrections 

 or changes of colour in those parts of the Government Survey which 

 relate to the rocks described in the previous sections. And before 

 leaving this subject, I may add my present conviction, that the 

 Coniston grits will turn out to be the representatives of the May Hill, 

 rather than of the Caradoc, sandstone. 



Admitting the previous statements, some not unimportant con- 

 clusions seem inevitably to follow from them : — 



(1.) In the Second Fasciculus of the description of the Palaeozoic 

 fossils of the Cambridge Museum, there is given an enumeration of all 

 the species below the Old-red-sandstone, with the per-centage of species 

 supposed to be common to the Cambrian and Silurian rocks, all the 

 new species being determined on the authority of Professor M'Coy, 

 and all the other species having had the sanction of his careful exa- 

 mination and description. Now, in making out this per-centage, I 

 had accepted the interpretation of the Government Survey, so far as 

 regarded their Caradoc sandstone ; and I had consequently counted 

 as Lower Silurian in their nomenclature (or as Cambrian in my own 

 nomenclature), many rocks (such as the Malvern and JVIay Hill 

 sandstone, &c.) which I now believe to be true, or Upper, Silurian. 

 Hence, in my present view, the per-centage of species, which are 

 common to the true Cambrian and true Silurian rocks, must be con- 

 siderably reduced ; and we shall now have no real difficulty (even if 

 we adopt the word System) in affirming that, below the Old-red- 

 sandstone, we have in Britain, at least, two palseontological systems, 

 by whatever names it may hereafter be thought right to designate 

 them. 



* See Quart. Jouin. Geol. Soc. vol. i. p. 17. 



