Oh. XXVIL] SUBDIVISIONS OF SILURIAN ROCKS. 



' LOWER SILURIAN ROCKS. 



551 



Caradoc. 



1. Caradoc Formation. 



Caradoc Sandstone. — Shelly "] 

 sandstones and conglomerates j 

 and shales. 



Bala Limestone.— Arenaceous 

 limestone ; slate, and sand- 

 stones with trappean tuffs. 



12,000 



r BracMopoda, numerous ; 

 Lamellibranchiata ; Ce- 

 phalopoda ; Pteropoda 

 {Conularia) of large size; 

 Cystidese, abundant ; 

 Trilobites, reaching their 

 maximum in species ; 

 Graptolites numerous. 



2. Llandeilo Formation. 



Upper 

 Llandeilo. 



Lower 

 Llandeilo. 



r« 



a. Upper Llandeilo. — Dark-col- 

 ored slates, with calcareous 

 flags and sandstones. 

 Lower Llandeilo or Arenig - 

 Beds.— Quartzose sandstones 

 and grits, with argillaceous 

 slates. 



Volcanic Rocks contempora- ~\ 

 neons with a and b. — Strati- | 

 fled tuffs (3300 ft.); felspathic V 

 and porphyrinic lavas (2500 

 ft.). 



r Mollusca, chiefly Cepha- 

 lopods of large size ; 

 Heteropoda (Bellero- 

 phori) numerous ; Grap- 

 tolites ; Trilobites of 

 large size. 



f Fossils of the same genera, 

 but all differing in species 

 J from those of the Upper 

 Llandeilo. Trilobites nu- 

 merous ; Graptolites of 

 various species. 



r 



1000 



to 

 1500 



i Organic remains, as in a 

 and o. 





The name of Silurian was first proposed by Sir Roderick Mur- 

 chison for that great series of fossiliferous strata which lie immediately 

 below the Old Red Sandstone, and occupy that part of Wales and 

 some contiguous counties of England which once constituted the 

 kino-dom of the Silures. a tribe of ancient Britons. 



UPPER SILURIAN ROCKS. 



1. Ludlow Formation. 



This member of the Upper Silurian group, as will be seen by the 

 above table, is about 800 feet thick, and subdivided into two parts— 

 the Upper Ludlow and the Lower Ludlow — at or near the top of 

 which last occurs the Aymestry limestone. Each of these may be 

 distinguished near the town of Ludlow, and at other places in Shrop- 

 shire and Herefordshire, by peculiar organic remains. 



Upper Ludlow. — a. Downton Sandstone. — This uppermost sub- 

 division was originally classed by Sir R. Murchison, under the name 

 of " Tilestones," with the Old Red Sandstone, the beds being often of 

 a similar red color. The whole was regarded as a transition group 

 forming a passage from the Silurian strata to Old Red Sandstone ; but 

 it is now ascertained that the fossils agree in great part specifically, 

 and in general character entirely, with those of the underlying Upper 

 Ludlow rocks. Among these are Orthoceras bullatum, Platyschisma 



