m 



SILURIAN SYSTEM. 



SILURIAN SYSTEM. 



T71 



situations, M tit* Caradoc Hills, where compact felspar predominates ; 

 the Wrekin aud LiUeshall H ill, characterised by .ienitic rock. ; Corndon, 

 foil of greenstone. Alteration* at itntiBed roolu by the contact of 

 igneous look* are oomtuou in the Caradoc, Stipentones, 4c. The tnp 

 rock* near Welshpool are in plaoee columnar ; the Breiddyn Hill* are 

 mostly greenstone, and yield elongated dyke* in a north-cut direction, 

 which tmrene the new red-sandstone. Mineral vein* (yielding lead- 

 ore) are plentiful in Lower Silurian rocks, in the Shelve district, 

 adjacent to the trap rock* of Corndon and the altered sandstone* of 

 the Stipentones. " In a plan of Mr. More'* of Linley Hall, the chief 

 proprietor of this district, upward* of 24 are laid down in the district 

 of Shelve alone, excluding the tract* around the Bog and Penally ; so 

 that, comprehending the principal portion of the mining-ground, we 

 may aay that it contain* upward* of 30 metalliferous vein* which hare 

 hem profitably worked." (Murehuon, Silurian System,' p. 282.) 



Volcanic grit*, composed of material* derived from igneous action, 

 and subsequently arranged in water, are mentioned by Sir Roderick 

 Murchison rather frequently. In the Shelve district they are traversed 

 by lead veins ; in the Caradoc Hill* tbry abound, and were noticed 

 as ' allied to greenstone' in the Wrekin by Mr. A. Aikin. They contain 

 organic remains in several place*, as near the Corndon Hills. 



On reviewing the series of strata comprised in the Silurian System 

 in the vicinity of Ludlow, Uak, Llandeilo, or Denbighshire, we see 

 them to form in reality one closely-associated sequence of oceanic 

 depoaite apparently accumulated with little local disturbance and 

 vary slight admixture of organic exuvite from the land. Volcanic 

 eruption* appear to have rather varied than greatly disturbed this 

 system of operation?, though it is evident they contributed no small 

 part of the granular materials of the principally sedimentary strata. 

 The formation of limestone is local: where coral prevailed, we find 

 the Aymestry and Wenlock limestones, and even the calcareous parts 

 uf the Landeilo rocks, to be in a great degree filled with coral The 

 Brachiopod shell fentamena fills some whole beds of limestone 

 (near Aymestry), and where it is deficient the limestone also fails, as 

 in the district of Usk. In their course from Shropshire northward to 

 Denbighshire, Mr. Bowman (' Reports of the British Association for 

 1840-41') has found the general type of the Silurian rocks to vary, 



and the line of distinction between it and the slaty strata below to be 

 extremely obscure; and similar observations are recorded by Sir 

 Roderick Murcbiaou in the account which he give* of these rocks in 

 Caeruiartheiubire and Pembrokeshire. 



Mineral character alone will scarcely suffice, anywhere, for any but 

 an arbitrary (and therefore unsatisfactory) boundary-line between the 

 Silurian and Cambrian deposit*. It is extremely probable, perhaps 

 we may say it is already proved, that no distinction of higher value 

 can be found on comparing the organic remains of these groups. In 

 Snowdon (supposed to be very low in the Cambrian *eries of rocks) 

 are shells and corals, which are perhaps the same, but certainly are 

 congeneric with and very similar to Silurian fossils; and there is 

 really as great (if not greater) difference between the Landeilo and 

 Wenlock rocks, in regard to fossil*, than between the Silurian and 

 Cauibri&n strata. 



If we turn to other districts where Silurian fossils occur plentifully 

 (North America, Ireland, Norway), the result appears the same. There 

 is apparently only one great series of organic combinations distinguish, 

 able among the foasiliferou* strata anterior to the old rad-saudstone 

 era, and it was with a perception of this important truth that Sir 

 Roderick Murchison once proposed for the Silurian strata the title of 

 Protozoic. If instead of this we employ Palaeozoic (as suggested by 



j Sedgwick), we shall rank all the fossiliferous strata of the Cumbrian. 

 Cambrian, and Silurian groups as Lower Palaeozoic Strata. 



The lower arbitrary boundary of the Silurian Strata being thus 

 softened or erased, we may regard its upper surface as only locally 

 more definite. Certainly in all the region around Wales the separation 

 of the Silurian and old red-sandstone deposits is somewhat sudden : the 

 colour changes from gray to red ; the dull mudstones become micaceous 

 sandstones ; the richly fossiliferous Upper Ludlow loses its character 

 in unprolific red marls and grits. What few fossils do occur in these 

 overlaid strata (except near the very bottom) arc of quite other types 



i of organisation. But these are local truths, depending mainly on the 

 introduction of new sediments poisonous to marine invcrtebral life ; 

 and as these sediments are very local, we may find in other countries 

 groups of strata newer than the Silurian, older than the Carboniferous, 



i with fossils intermediate in character and combination to both. 



The following analysis from Professor Phillips' s ' Manual of Geology,' gives a more extended view of these rocks : 

 Collective Titles. Formations. 



Ludlo 



A : * 



Tnuuition Group 



Bala or Uandeilo 



Festiaiog or Lingula 



Bangor or Longnij nd 



On considering the distribution of organic remain* in the successive 

 stages of the Silurian rocks, it i* evident that the greatest variety of 

 specie* occur* in the lower part of the Upper and toward* the upper 

 part of the Lower Silurian rocks. In other words, the condition* 

 favourable to organic life in the sea were in the earliest period con- 

 siderable ; they arrived st a maximum in the middle part of the period 

 in the Caradoc sandstone, the Wenlock shale, and the Wenlock lime- 

 stone, and U11 continued considerable till the Silurian depositions 

 ceased, and were replaced by old red-sandstone nearly devoid of 

 organic remains. Polyinaria, Crinoidia, and Cnulacca an most nume- 

 rous in the principal calcareous rock, Weulock limestone; Brachiopoda 

 are most plentiful in Caradoc sandstone ; Cephalopoda in the Weulock 

 i ; flahes in the upper Ludlow rock. 



Prevalent 

 Mineral Type. 



; ( Tilestone Arenaceous. 



| ) Upper Ludlow Argillo-Arenaceous. 



. . Calcareous. 



m m } Aymcjtry Limestone 

 * - \ Lower Ludlow 



5 5 ( Wenlock Limestone 



1 { Wenlock Shale 

 _g | Woolhope Limestone 



Uay II1U or Upper Caradoc Saudstoue 



J { Caradoc Sandstone 



g ( Bala Limestone 

 1 S | Bala Slate Flag* . 



Argillaceous. 



Calcareous. 



Argillaceous. 

 Calcareous. 



Arenaceous. 



Variable. 



Variable. 

 Variable. 



) . ( Arcing Slate and Porphyry . . . Argillaceous. 



j Trrmudoc Slate Argillaceous. 



* ( Lingula Flag* Argillaceous. 



-S p ( Hcnlcch Grits 

 I { Llanbertis Slates and Grits . 

 J5 ( Longmynd Slates 



'S* \ 



Arenaceous. 



Argillo-Areniceoiu. 



Argillo-Arenaceotu. 



ZOOPHTTA. 



upper Ludlow 

 The following list of fossils is given by Professor Phillip* : 



A 

 CAoria 



iWolAyra 



AUOBPHOZOA. 

 Sptd**. 

 (0 . .1 Vntmidium. 



. . 2 



FOBAXWITEHA. 



>; . . 



i 



. i 



Actroploria 

 Alteolilct 

 ArochnophyUum . 

 AtUacopkylltun 

 Aulopora . 

 Ctuetitei . 

 Cladocora . 

 ClinophyUwn . 

 Ctmiitt 



Cyatkajronia . 

 dyathophyllum . 

 Oyttiphyllum . 

 IMphypkyt!** . 

 Fanonta 

 fattUipora . 



fKdymograpnu . 

 Diployrapiiu . 

 Qorgonia 



(Zoantharia of Edwards.) 

 Species. 



Ooniophyllum . 



Jlalyiilet 



JldMita 



Pctraia 



I'yrilonema . 



Protovirgularia . 



SnrcituUa 



Stcnopora . 



Strepkodet 



>' MMM ... | 



Stronbodc* 



Syringopora 



Zapkrenla . 



ALCYONARIA. 

 Species. 



. S (JraptrjlMuf 

 . 10 Ra&rUtt 

 . 4 Rctiolilu 



. 2 

 . 1 

 . 10 

 . 7 

 . 1 

 . 1 

 . 1 

 . 1 

 . 5 

 . 2 

 . 2 

 . 6 

 . 2 

 . 2 



SpseV . 

 . 12 

 . 1 



1 



