430 



SUBDIVISIONS OF SILUEIAls ROCKS. [Ch. XXVIL 



1. Ludlow . 

 formation. ' 



2. Wenloek. 

 formation. 



UPPER SILURIAN" ROCKS. 



Prevailing Lithological J^'^^" 

 characters. Feer 



Upper 

 Ludlow. 



Aymestry 

 limestone. 



Lower 

 Ludlow. 



Wenloek 

 limestone. 



"Wenloek 

 shale. 



a. Tilestones. — 

 Finely lamina- 

 ted reddish and 

 green micaceous 

 sandstones. 



b. Micaceous gray "" 

 sandstone and 

 mudstone. 



Argillaceous lime- 

 stone. 



Shale, with concre- 

 tions of lime- 

 stone. 



Concretionary and 

 thick-bedded 

 limestone. 



Argillaceous shale, 

 frequently flag- 

 stone. 



800? 



!000 



Above 

 2000 



J 



Organic 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 land plants. 



Marine Mollusca of 

 various orders as 

 before. Crinoidea 

 and corals plenti- 

 ful. Trilobites, 

 Graptolites. 



Caradoc 

 formation 



MIDDLE SILURIAN ROCKS. 



r Shale, shelly lime- ") 

 j Caradoc J stone, sandstone, I oooo 

 1. I sandstones. 1 and conglome- f 

 I rate. j 



Crinoidea, Corals, 

 Mollusca, chiefly 

 Brachiopoda. 

 (The genusPenta- 

 merus abundant.) 



LOWER SILURIAN ROCKS. 



Llandeilo j Llandeilo 

 formation, ) fiay-s. 



Dark colored cal 

 careous 



slates and saud 

 stones. 



I cal- "1 f Mollusca, Trilo- 



fl-'^gs; 120 000 J l^itos, CysJ^idese, 



20,000 



i Cri 

 I Gri 



noids, Corals, 

 aptolites. 



UPPER SILURIAN ROCKS. 



Zudlo2(j formation. — This member of the Upper Silurian g^'oiip, 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. Ujjper 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- 



