430 



SUBDIVISIONS OF SILURIAN ROCKS. [Ch. XXVIL 



UPPER SILURIAN ROCKS. 



I. Ludlow 

 formation. 



2. Wenlock. 

 formation. " 



Prevailing Lithological 

 characters. 



a. Tilestones. — 

 Finely lamina- 

 ted reddish and 

 Upper green micaceous 



Ludlow. ■ san <istones. 



b. Micaceous gray 

 sandstone and 

 mudstone. 



Aymestry j Argillaceous lime- 



limestone. 



Lower 

 Ludlow. 



stone. 



Shale, with concre- 

 tions of lime- 

 stone. 



Thick- 

 ness in 

 Feet 



■ 800 » 



2000 



i Concretionary and 



v , ■< thick-bedded 



limestone, J v . 



limestone. 



Wenloek j Argillaceous shale, 

 •{ frequently flag- 



shale. 



( stone. 



Above 

 2000 



ic remains. 



Marine mollusca of 

 almost every or- 

 der, the Brachio- 

 poda most abun- 

 dant. Serpulites, 

 Crustaceans of 

 the Trilobite fa- 

 mily. Placoid 

 fish (oldest re- 

 mains of fish yet 

 known). Sea- 

 weeds ; and in 

 the uppermost 

 strata landplants. 



Marine Mollusca of 

 various orders as 

 before. Crinoidea 

 and corals plenti- 

 ful. Trilobites, 

 Graptolites. 



MIDDLE SILURIAN ROCKS. 



f Shale, shelly lime- 

 Caradoc ( Caradoc 6tone, sandstone, 



formation. ( sandstones. ] and conglome- 



[ rate. 



2000 



Llandeilo 

 formation. 



LOWER SILURIAN ROCKS. 



Llandeilo 

 flags. 



Dark colored cal- 

 careous flags 

 slates and sand 

 stones. 



a 



'Crinoidea, Corals, 

 Mollusca, chiefly 

 Brachiopoda. 

 (The genusPenta- 

 merus abundant.) 



{Mollusca, Trilo- 

 bites, Cystidese, 

 Crinoids, Corals, 

 Graptolites. 



UPPER SILURIAN ROCKS. 



Ludlow formation. — This member of the Upper Silurian group, as 

 will be seen by the above table, is of great thickness, and subdivided 

 into three parts, — the Uppe : and the Lower Ludlow, and the intervening 

 Aymestry limestone. Each of these may be distinguished near the town 

 of Ludlow, and at other places in Shropshire and Herefordshire by pe- 

 culiar organic remains. 



1. Upper Ludlow, a. Tilestones. — This uppermost subdivision, called 

 the Tilestones, was originally classed by Sir R. Murchison with the Old 

 Red Sandstone, because they decompose into a red soil throughout the 

 Silurian region. They were regarded as a transition group forming a 

 passage from Silurian to Old Red ; but it is now ascertained that the 

 fossils agree in great part specifically, and in general character entirely, 

 with those of the underlying Silurian strata. Among these are Ortho- 



