Ixxii PROCEEBINGS OF THE GEOLOGICAL SOCIETY. 



of great interest, because it appears to be associated with that great 

 change in organic hfe which constitutes the transition from the 

 Lower Silurians of Sir R. Murchison or the Cambrians of Prof. 

 Sedgwick, to the Upper Silurians of the former geologist. If we 

 draw the hne of demarcation between these two great divisions of 

 the Palceozoic rocks immediately above those beds which have been 

 generally regarded as the upper of the Caradoc beds, we have a con- 

 siderable number of organic forms common to the Upper and Lower 

 Silui'ians. This community of character in the organic contents of 

 these rocks has been particularly noticed by Mr. Phillips in the 

 Malvern district, and the higher of the beds considered as Caradoc 

 in N. Wales have been observed to contam many Wenlock species ; 

 but we owe to the accurate and acute observation of Prof. M'Coy 

 the suggestion that the proper line of demarcation between the Upper 

 and Lower Silurian groups ought probably to be drawn between the 

 beds hitherto considered as Upper Caradoc, but containing many 

 Wenlock species, and the subjacent beds which are distinguished by 

 Lower Silurian fossils and constitute the true typical Caradoc. The 

 Caradoc beds of May Hill, those which wrap round the Berwyns on 

 the north and north-west, and those which lie on the boundary of 

 the Longmynd, belong to the first of these groups. The subject, as 

 you will recollect, was recently brought before us by Prof. Sedgwick, 

 and the testimony of Prof. M'Coy affords good reason to believe that 

 the per-centage of species common to the Upper and Lower Silurians 

 \n\\ be considerably reduced by adopting this new plane of separation 

 between them. Should this be the case, it will be a valuable step in our 

 geological classification, bringmg the phsenomena in our own country 

 into more perfect coordination wdth the corresponding pheenomena 

 of N. America and Bohemia, where the number of species common 

 to the two great divisions of the Palaeozoic rocks of which we are 

 speaking, is much less than it has hitherto been considered to be in 

 these islands. Mr. Hall remarks*, " Commencing with the lowest 

 rocks known to contain fossils, we find the first important change in 

 the typical forms to occur at the termination of the Hudson-river 

 group ; which is marked by a coarse sandstone or conglomerate 

 (the Oneida conglomerate or Shawangunk grit), beyond which 

 scarcely a single species has prolonged its existence. This point 

 must be considered as representing that horizon Avhich, in Great 

 Britain, is the termination of the Lower Silurian deposits." M. 

 Barrande divides the Silurian rocks of Bohemia into two great 

 divisions, corresponding to the Upper and Lower Silurians of Sir 

 R. Murchison. Their separation does not appear to be marked by 

 non-conformity in the stratification, but by a great overflow of 

 igneous matter. This Trappean mass alternates with argillaceous 

 schistose beds containing about twenty species of Graptolites, identi- 

 cal with those found in beds of a similar character at a considerably 

 greater depth in the series. The same beds contain also about 



* Palaeontology of New York, vol. i. Introduction, p. xvi. 



