136 
SILUEIA. 
[Chap. VII. 
may be consulted both in the ' Silurian System ' and in Professor Phillips's 
monograph *. 
A very remarkable section of the Passage-beds from the Upper Silurian 
Rocks into the Lower Old Red Sandstone has been laid open on the line of 
Railway between Worcester and Hereford, near the town of Ledbury. It 
was described by the Rev. W. S. Symonds, in full detail, in the Quarterly 
Journal of the Geological Society, vol. xvi. p. 194 ; and vol. xvii. p. 152 &c. 
The section was more perfect than that at Ludlow, not being interfered 
with by any break in the strata. The Aymestry rock (with Pentamerus 
Knightii) occurred in the Ledbury tunnel ; and the rocks in ascending order 
to the point opposite the Ledbury Station furnish the following series of 
strata and fossils: — 1. Aymestry rock (10 feet). 2. Upper Ludlow rock 
with Chonetes lata &c. (140 feet) ; Ludlow bone-layer not detected. 
3. Downton sandstone with the characteristic Lingula ; 9 feet. 4. Red 
and mottled marls and thin sandstones, with Lingular and remains of 
Pteraspis ; 210 feet. 5. Grey shales and grit at the entrance of the tunnel, 
with Cephalaspis Murchisoni and Pterygotus ; 8 feet. 6. Purple shales 
and thin sandstones (34 feet). 7. Grey marl passing into red and grey 
marl and bluish-grey rock (Auchenaspis-grits), with Auchenaspis, Plec- 
trodus, Cephalaspis (two species), Onchus, Pterygotus, a Lituite, and 
Lingula ; 20 feet. These strata pass conformably upwards into a series 
of red marls, with grey and reddish sandstone, in which Henry Brooks, the 
geological shoemaker of Ledbury, found the remains of Pteraspis and 
Cephalaspis. It was owing to his persevering labour that Mr. Symonds 
was happily enabled to correlate these beds with their equivalents near 
Ludlow. It was thus found that fossils diminish as soon as the horizon of 
the Upper Ludlow rocks is passed ; but, according to Mr. Salter, the Lingula 
of the Downton beds is the same as that found at the base of the Old Red. 
Again, there is little doubt that the Pteraspis of the Bone-bed ascends into 
the Lower Old Red, where it is associated with Cephalaspidian Pishes. 
Whilst it is difficult to define with minute accuracy the line which sepa- 
rates the lower system from the upper, it is clear that the Ledbury sec- 
tion proves the value of the term ' Passage -beds' even more fully than the 
sections near Ludlow, Kington, &c. 
Similar passages upwards, from inferior grey-coloured to superior red 
rocks, are observable at Usk and in tracts around that town, where the 
Ludlow rocks rest upon Wenlock limestone f and are full of the ordinary 
fossils. This small Silurian oasis in a region of Old Red Sandstone (see 
Map) is highly interesting, and exhibits on either bank of the River Usk 
some peculiarities of mineral structure connecting the eastern and western 
tracts of the Silurian region. 
In Brecknockshire, to the south of Builth, the Ludlow rocks, surmount- 
ing a noble escarpment of the lower member of the Upper Silurian on the 
* Mem. G-eol. SurT. Great Brit. vol. ii. part 1. t Sil. Syst. p. 408. 
