196 



UPPER SILURIAN ROCKS — LUDLOW FORMATION. 



vision of rocks being characterized by a corresponding suite of organic remains), was 

 formerly considered one assemblage, without definite sequence, and included under 

 the unmeaning names of greywacke or transition limestone 1 . I have already ex- 

 plained that the latter term has been as liberally bestowed (chiefly, however, by 

 foreigners) upon the carboniferous limestone, from which the Silurian rocks are se- 

 parated by that enormous accumulation, the Old Red Sandstone ; whilst the organic 

 remains of both these systems are entirely dissimilar from those of the carboniferous 

 sera. Referring to the introductory chapter for further explanation, let us now pro- 

 ceed to consider these Silurian deposits in the natural order in which they appear in 

 the south-west of Salop and adjacent parts of Hereford. 



Llandeilo Formation. Caradoc Formation. Wenlock Formation. Ludlow Formation. 22. 



Llandeilo Caradoc Sandstones. Wenlock Wenlock Lower Aymestry Upper Tilestone of 



Flags. (g. and i. impure limestones.) Shale. Limest. Ludlow. Limest. Ludlow. Old Red. 



Lower Silurian Rocks. Upper Silurian Rocks. 



Upper Silurian Rocks. — 1st formation, ' Ludlow Rocks'. (PL 31. fig. 2. and 



I.e. d. oi this wood-cut.) 



The grey-coloured strata to which the name of "Ludlow rocks " has been applied, rise 

 from beneath the Old Red Sandstone into separate mountain ridges and occupy a large 

 portion of the area distinguished in the map by the light purple colour 2 . Examples of 

 this order of infraposition are exposed in numberless natural sections which cross this 

 zone of rocks between the hills near Ludlow on the north-east, and the sea-cliffs consti- 

 tuting the south-western extremity of Pembrokeshire, a distance of about one hundred 

 and fifty miles. The term of " Ludlow rocks" has been selected, because the town of 

 Ludlow is built upon the upper beds, near their junction with the Old Red Sand- 

 stone ; the neighbourhood affording great facilities for studying the other divisions of 

 the formation, as well as those older deposits which succeed beneath it. To the west, 

 north-west, and south-west of Ludlow, these rocks rise into hills, the highest of which 

 are about eleven hundred feet above the sea. The uppermost strata slope down into 

 the lower country of Old Red Sandstone, while the inferior beds are well exposed in 

 deeply channelled and broken escarpments. By this arrangement of the strata, and 



1 See the first, or Introductory Chapter. 



2 The formations composing the Silurian System are distinguished on the map by different letters, as well 

 as by different colours. The letter n marks the Ludlow formation ; viz. n. Upper Ludlow ; ri. Aymestry lime- 

 stone ; n". Lower Ludlow. The letter o marks the Wenlock formation ; viz. o. Wenlock limestone ; of. Wen- 

 lock shale. The Caradoc formation is indicated by p ; the Llandeilo by q. The double letters n + o represent 

 the Upper Silurian Rocks, and p + q the Lower Silurian, where the subdivisions cannot be traced. 



