Chap. VII.] 
UPPEE LUDLOW ROCK. 
131 
the Shells more characteristic of the next member (the Upper Ludlow 
rock), this limestone is not greatly distinguished by its fossil contents from 
the Wenlock limestone. 
In some tracts, as at Usk, Monmouthshire, the place of this rock is 
marked only by the shelly courses which near Ludlow form its immediate 
cover; and wherever these peculiar beds occur, they are replete with 
Ehynchonella navicula (PI. XXII. f. 12) and the small Leptasna laevigata *. 
It is this band which forms the base of the Upper Ludlow rock ; and it 
may be well seen in the quarries at Aymestry. 
Upper Ludlow Rock. — This is the most diversified, in structure and 
contents, of the three subdivisions of the highest Silurian formation, and 
is also remarkable in exhibiting a transition into the next overlying 
system, the Old Eed or Devonian. Its lowest stratum is the calcareous 
shelly band, charged with Hhynchonella navicula, which has just been 
mentioned as forming the roof of the Aymestry limestone, and which occa- 
sionally attains a thickness of 30 or 40 feet. This is surmounted by grey, 
calcareo-argillaceous masses, so common throughout the Silurian rocks, and 
which, from their incoherent nature, easily decompose into mud. Like 
other Silurian sediments of higher antiquity, this mudstone has a ten- 
dency to run into large spheroids, and occasionally contains small concre- 
tions of sandy clay, which, being more destructible than the pure argil- 
laceous matrix, weather out in the faces of the escarpments, marking the 
lines of stratification by small elliptic cavities like swallow-holes. 
The chief and distinguishing portion of the Upper Ludlow contains more 
calcareous matter and sand than the beds immediately beneath. It is, on 
the whole, a slightly micaceous, thin-bedded stone, of bluish-grey colour 
within, but weathering externally to a brown, rusty grey, and remarkable 
for its symmetrical, transverse joints, as exhibited at a, a in the following 
view (p. 132). 
Some strata of this character appear also in the foreground of the 
vignette, p. 123, which represents the Castle of Ludlow standing on the 
rock out of which it was built. Though quarried extensively for use, the 
stones, either when not well selected, or not placed horizontally in the 
wall in the direction of the layers, are very prone to decomposition. This 
rock is also well exhibited near Ledbury. 
I have elsewhere compared this member of the Upper Ludlow with the 
< Macigno ' of Italy, and particularly with those dark-greyish portions of 
the latter which occur between Perugia and Florence. This lithological 
resemblance of so old a rock to so young a deposit (for the Italian Macigno 
is no older than the London Clay) is cited to show how similar materials 
— sand, mud, and calcareous matter, collected on a sea-bottom, often 
necessarily resemble each other when formed into stone, though originally 
deposited at such very widely separated periods. In this comparison it is 
* Mem. Geol. Surv. toI. ii. pt. 1. pi. 26. fig. 3 (L. lepisma). 
K 2 
