246 C. SCHUCHERT — LOWER HELDERRERG-ORISK A N V FORMATIONS 



Silurian locks woe first classified and described, we find them diversified by in- 

 terpolated courses of limestone ; much calcareous matter being also disseminated, 

 both in nodules and in flagstones. 



" With additions like these to the richness and variety of the subsoil, which are 

 so welcome to the proprietor and farmer, the geologist usually discovers a much 

 greater abundance of fossil animal remains than in the same strata of the sterile 

 western tracts. By observing the older of superposition, and by tracing the di- 

 visionary limestones, he reads off the order of the strata, and chronicles with pre- 

 cision the succession of their respective fossils. 



" In this way the Upper Silurian rocks are seen to consist, as a whole, of the 

 two formations to which I assigned the names of Wenlock and Ludlow, each, of 

 these being subdivided in the manner expressed in this woodcut."* 



Murchison then describes the various horizons, and names the diag- 

 nostic fossils for the Upper Silurian. Regarding the highest beds of the 

 Ludlow, he states that — 



" The Upper portion of the Ludlow formation, or capping of the bone bed is com- 

 posed of light colored, thin bedded, slightly micaceous sandstones, in which quar- 

 ries are opened out near Downton castle on the Teme.| The uppermost layers of 

 the whole system, and which form a transition into the Old Red sandstone, con- 

 sist of tilestones and sandstones, occasionally reddish, in which, besides other 

 fossils found in the Ludlow rock, the Lingula cornea, with crustaceans [Lepto- 

 cheles], and defenses of fishes, often occur. 



" Being compelled in my researches to draw a line of demarkation between the 

 upper part of the Ludlow formation and the bottom of the overlying Old Red sand- 

 stone, I formerly included the tilestones in the latter ; particularly as in most parts 

 of the region they decompose into a red soil, and thus they afford a clear physical 

 line of demarkation between them and the inferior rocks, which facilitated the 

 construction of the geological map. 



"Even then, however, the fossils which were figured as characteristic of such 

 tilestones exhibited little else, as I showed, than species common to the Ludlow T 

 rock itself. This zoological fact, and subsequent researches in other parts of Eng- 

 land, above all those of Professor Sedgwick in Westmoreland, where the Upper 

 Ludlow strata are much developed, have, for eleven years, led me to classify these 

 tilestones with the Silurian rocks, of which they form the natural summit. [The 

 fossils are listed and figured on plate xxxiv.] All of these are the most common 

 fossils of the Upper Ludlow rock ; although a few of them descend as low as the 

 Caradoc sandstone." i 



With the exception of the fishes, the fauna included in these tilestones, 

 and listed by Murchison, is in harmony with the American Upper Silu- 

 rian. It does not suggest anything higher than the Niagara group. On 



*Siluria (pp. 101-102). The legend to this woodcut forms the first column of the correlation 

 table beyond. 



fThe Downton Castle stone. Silurian system, p. 197. 

 X Ibid., pp. 138, 139. 



