and the Palceozoic System of England, SI 7 



deposits violent mechanical movements and shifting conditions, 

 gradually produced an extinction of many of the older and 

 most characteristic organic types. And in the discordant posi- 

 tion of the overlying May Hill sandstone, we have not only the 

 proof of great anterior mechanical movements among the upper 

 Cambrian groups, but in it we find the commencement of a new 

 succession of the regular Silurian deposit ; and along with this 

 new physical succession we find a co-ordinate change of the 

 organic types. For in the May Hill sandstone the old cha- 

 racteristic organic types have suddenly disappeared ; and in their 

 place we have another set of types, which are characteristic of 

 the lower groups of Siluria. In short (to use a language now 

 in common use), we have in this way the introduction of a new 

 " system,^^ physically and palseontologically distinct from the 

 whole system of Cambria. 



Taking the May Hill sandstone as the base of the Silurian 

 groups, I venture to affirm that no two subdivisions of the whole 

 British palseozoic series are better defined, by physical characters, 

 than are the collective Cambrian and Silurian groups. So far 

 as regards the organic remains, I may remark that in the North 

 of England, if all the fossil-bearing rocks below the Old Red 

 Sandstone be divided into two groups, the upper group ending 

 with the equivalent of the May Hill sandstone, out of about 

 180 well ascertained species of fossils, we do not find more' 

 than 3^ per cent, of such species as common to both groups. 

 The number of species common to the Cambrian and Silurian 

 rocks, as they are developed in Wales and the bordering coun- 

 ties, is considerably greater ; probably amounting to seven or 

 eight per cent*. Much of the confusion between Cambrian 

 and Silurian types has not been the work of Nature's develop- 

 ment, but has been introduced by erroneous sections, and an 

 erroneous grouping of the May Hill sandstone with the upper 

 sandstones of the great Bala group. In conformity with the 

 title of this paper, I next proceed to give a tabular view of the 

 whole palaeozoic system of England. 



* A much larger per-centage of common species may be made out of 

 the tables appended to the 2nd Fasciculus of the Cambridge Palaeozoic 

 Fossils. But when proper corrections have been made by the introduction 

 of the May Hill sandstone as the true base of the Silurian series, and by 

 the elimination of those localities which, in the field, give no sectional evi- 

 dence of their true geological place. Professor M'Coy does not think that 

 the common species will form a larger per-centage than that above given. 



