OF SILURIAN ROCKS. (Ca. XXVIL 430 SUBDIVISIONS

UPPER SILURIAN ROCKS. Thick Prevailing Llthological ness In Organic remains. characters. FeeL

fa. Tilc.tonC3.-1 Finely lamina- Marine moihisca of ted reddish and 800! almost every or r Brachio- green micaCCOUS der, the Upper sandstones. poda most Ludlow. abunSerpulites,-dant. b. Micaccous gray Crustaceans of the Trilobite fa I. Ludlow I sandstone and 1 Placoid formation. ( mudstone. mily. fish (oldest re lime200 mains of fish Aymestry Argillaceous 0 yet limestone. stone. known). Sea weeds; and in the ( Shale, with concre- uppermost tions of lime- strata land plants. stone.

C and Wenlock Concretionary Marine Mollusca of thick-bedded I various orders as limestone. limestone. 2. Wenlock. Above before. Crinoidea 2000 and corals formation. shale, I plenti \Tenlock ( Argillaceous fuL Trilobites, ( I ehaic. frequently flag- Graptolites. ( stone. j J

MIDDLE SILURIAN ROCKS. Crinoidea, Corals, (Shale, lime-' shelly Mollusca, Caracloc ( Caradoc I stone, sandstone, chiefly 2000 . Brachiopoda. formation. sandstones. 1 and conglome I rate. I (The genusPenta nierus abundant.)

LOWER SILURIAN ROCKS.

(Dark colored cal- (Mollusea, Trilo Liandeilo ( Liandeilo I careous bites, flags; 1,,. 20,000 Cystidete, formation. flags. 1 slates and saud- 1 Crinoids, Corals, stones. I.. Graptolites.

UPPER SILURIAN ROCKS. Ludlow as formation.-This member of the Upper Silurian group, will be seen the by above table, is of great thickness, and subdivided into three parts,-the Uppo and the Lower Ludlow, and the intervening limestone. Aymestry 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. Tilcstones.-_This uppermost subdivision, called the Tzlestone8, was originally classed by Sir R. Murchison with the Old Red Sandstone, because they decompose into a red soil throughout time Silurian a region. They were regarded as a transition group forming 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 Ortlio-