SILURIAN SYSTEM— LUDLOW FORMATION. 



197 



their varied directions caused by elevations to which we shall presently allude, the 

 formation can be studied in detail, and hence we learn that it is divisible into three 

 parts, each distinguished by characteristic organic remains. These subdivisions I have 

 termed 



1 . Upper Ludlow Rock. 



2. Aymestry Limestone. 



3. Lower Ludlow Rock. 



Although a common lithological aspect and colour pervade the strata of the formation, 

 this triple subdivision is clearly observable throughout a large tract of Shropshire 

 and Herefordshire, extending in a mass of flexuous form, from Aymestry by Ludlow to 

 Onnibury, including the hills of Mocktree Forest ; and afterwards in a range of hills 

 lying between the red land of Corvedale and the straight ridge called Wenlock Edge. 

 The Ludlow rocks of this district will be first described, and then the older forma- 

 tions which emerge from beneath them. 



Upper Ludlow Rock (b of wood-cut). — This subdivision of the Ludlow formation 

 consists essentially of thin-bedded, lightly coloured, and very slightly micaceous sand- 

 stones, in some parts highly argillaceous, and in others so calcareous as to assume 

 the character of impure limestones. When deeply cut into, these beds are of greenish 

 grey or bluish grey tints, but they rapidly weather to an ashen or more rarely to a 

 rusty-brown colour. 



The upper beds, forming the downward passage from the Old Red System, are 

 yellowish sandstones, of a very fine grain and slightly micaceous. They are best 

 displayed at Ludford, Richard's Castle, and on both banks of the Teme near Downton 

 Castle (Tin Mill, &c), where they are extensively quarried for flags and wall- 

 building, and pass down into a greyish-coloured stone, of which Downton Castle is 

 built. 



Among the fossils of these upper beds is the small Lingula cornea, (PI. 3. fig. 3.), sometimes 

 having the nacre of the shell preserved, with fragments of carbonized vegetables, too imperfect to 

 be generically determined, but probably of terrestrial origin. Small, black, polished, round bodies, 

 the bufonites of old authors (portions of the palates of fishes), are not unfrequent. As the Lin- 

 gula cornea occurs in the tilestones of the Old Red Sandstone, these strata might be also as- 

 signed to the same System. They are, however, beds of passage, which cannot be arbitrarily re- 

 ferred either to the Old Red or Silurian Systems. This yellowish rock is exhibited in several other 

 parts of the district, as at Ashley Moor, Linton Lane, and North Field, near Mortimer's Cross, in 

 most of which situations it is observed dipping under the Old Red Sandstone, and graduating 

 downwards into the greenish grey strata of the Upper Ludlow rock. 



Sections at Richard's Castle expose these transition beds. The principal part of the village stands 

 upon a red micaceous sandstone, with much clay; and rising from beneath this into higher grounds 

 on the north-west, at an angle of about 18°, the following beds are successively laid open, either 

 in quarries or by the natural outcrop of the rock. 



2b 



