78 



ivith a gr^at admixture of Lower Silurian, not Upper Silurian, forms. 



'• The Ma J Hill Sandstone, on the contrary, as Sedgwick showed in 

 185o, is unequivocally the base of the Upper Silurian, and contains 

 scarcely any true Lower types." (Salter. Geological Magazine, vol. iii., 

 p. 240.) Now, the only deposit, as yet known in America, which ex- 

 hibits such an admixture, is Division 1 of the Anticosti group. If, then, 

 the extinction of the lower Silurian fauna occurred in the ancient British 

 seas at the same time that it did in the American waters, it follows that 

 Division 1 is Lower Llandovery ; and that the Hudson River is Caradoc. 



It is, however, very difficult to correlate all the divisions of the English 

 Middle and Lower Silurian with those of America, and I shall take this 

 ocLasion to make a few observations on the other members of the series 

 not found in Anticosti. From what we know of the origin and mode of 

 accumulation of sedimentary strata, it is highly improbable that each of 

 the minor formations of one country should have its exact equivalent in 

 another land several thousands of miles away, although the larger groups, 

 of which these smaller ones are the component parts, may be well repre- 

 sented, and paralleled in a general way. Everywhere we find a number 

 of breaks or gaps, and the probabilities are vastly against these breaks 

 having been all occasioned at the same time in distant localities. It is 

 more consistent with the nature^of things that many of the breaks in Ame- 

 rica should stand opposite — so to speak — the formations in England and 

 vice versa. Perfect parallelism of the minor groups maybe looked for as 

 the exception, not the rule.* 



Comparing the' Middle and Lower Silurian, I think we can identify, 

 with certainty, only two horizons in England and America. The upper of 



* Professor A. 0. Ri,m^ay gives the foUowiat? account of the breaks in the English 

 series from the Lingula Flags up to the Wenlock Shals : — 

 " Lingula Flags : 



Break very nearly complete both in genera and species, and probable uncon- 

 formity. 



Tremadoc Slate : 



Break very nearly complete, both in genera and species, and probable uncoa- 

 formity. 



Llandeilo and Caradoc beds : 



Large break, especially in species, and probable unconformity. 



Lower Landovery beds : 



Break and decided unconformity, 



Upper Landovery beds : 



Break and strong unconformity. 



Wenlock Shale," &c. 



(Ramsay. Anniversary Address to the Geological Society, 1863. Jour. Geo. 

 Soc, vol.xix.) 



