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CHAP. VI.
PALÆOZOIC STRATA.
139
the form and arrangement, offers us the best analogies. The coral islands of the South Seas are in this respect very dissimilar.
The best, or rather the only, complete British series of these rocks is that of the Welsh border, of which the above section is a sketch: below is Mr. Murchison's summary.
| Thickness. | Subdivisions. | Lithological characters. | |||||
| Upper Silu- rian. |
Ludlow Formation | 2000 | Upper Ludlow rock. | Slightly micaceous, gray coloured, thin-bedded sandstone. | |||
| Aymestry limestone. | Subcrystalline gray or blue argillaceous limestone. | ||||||
| Lower Ludlow rock. | Sandy, liver and dark coloured shale and flag, with concretions of earthy limestone. | ||||||
| Wenlock Formation. | 1800 | Wenlock, limestone. | Highly concretionary subcrystalline gray and blue limestone. | ||||
| Wenlock shale | Argillaceous shale, liver and dark gray coloured, rarely micaceous, with nodules of earthy limestone. | ||||||
| Lower Silu- rian |
Caradoc Formation. | 2500 | Flags. | Thin-bedded, impure, shelly limestone, and finely laminated, slightly micaceous, greenish sandstone. | |||
| Sandstones grits, and limestones. | Thick-bedded, red, purple, green, and white freestones; conglomeritic quartzose grits, sandy and gritty limestones. | ||||||
| Llandeilo Formation. | 1200 | Dark-coloured flags, mostly calcareous, with some sand stone and schist. | |||||
On a careful examination of the vicinity of Ludlow, all the upper parts of the Silurian rocks may be perfectly traced and clearly discriminated: it is in the vale of the Towey (Dinevawr) that the lower formation is
