254 MOUNTAINOUS COUNTRY OF THE SLATE ROCKS. 



19°. Lower Silurian. 3700 ft. The Caradoc beds consist of thicK 



Caradoc Sand-stones 2500 ^^y^^^ ^^ sand-stone of various colours, 

 Llandeilo ' Fl&gs 1200 ^^^^'^S "P?"' ^"^ <^oyejed by, and oc- 



° casionally interstratined with, thm beds 



of impure lime-stone. The Llandeilo 

 flags which he beneath them consist of 

 thin calcareous strata, in some locali- 

 ties alternating with sand-stones and 

 shales. 

 Extent. — These rocks form patches of land in Shropshire and the north of 

 Montgomery — and skirt the southern and eastern edge of Caermarthen. None 

 of the Silurian rocks have yet been found to extend over any large portion of 

 either Scotland or Ireland. 



Soil. — The Caradoc sand-stone, when free from lime, produces only a 

 naked surface or a barren heath. The Llandeilo flags form a fertile and arable 

 soil, as may be seen in the south of Caermarthen, where they are best devel- 

 oped, and especially on the banks of the Towey, which for many miles before 

 it reaches the town of Caermarthen runs over this formation. 



In this formation, as in every other we have yet studied, the soil changes im- 

 mediately on the appearance of a new rock at the surface. The soil of the 

 Wenlocic shale is sometimes more sandy as it approaches the Caradoc beds, 

 and on favourable slopes forms good arable land and sustains luxuriant woods, 

 but where the Caradoc sand-stones reach the surface, a wild heath or poor 

 wood-land stretches over the country, until passing over their edges we reach 

 the lime-containing soils of the Llandeilo flags, when fertile arable lands and 

 lofty trees again appear.* 



G. — Cambrian System. 



20°. Vjjper Sf Loiver Cam- ) These rocks, which are many thou- 



brian Rocks. \ ^ondi yards in thickness, con^st chiefly 



of thin slates, often hard and cleaving 

 readily, like roofing slates, occasionally 

 intermingled with sandy and thin lime- 

 stone beds. They contain few organic 

 remains. 

 Extent. — These rocks cover the whole of Cornwall, part of North and 

 South Devon, the western half of Wales, the entire centre of the Isle of Man, 

 and a large part of Westmoreland and South Cumberland. In Scotland, they 

 foiTH a band between 30 and 40 miles in width, which crosses the island from 

 the Mull of Galloway to St. Abbs Head. They form also a narrow stripe of 

 land, which recrosses the island along the upper edge of the old red sand-stone 

 from Stonehaven to the Isle of Bute, and, further north, spread over a consider- 

 able portion of Banffshire. In the south-west of Ireland they attain a great 

 breadth, are narrower at Waterford, but form a broad band along the granite 

 mountains from that city to Dublin. They extend over a large portion of the 

 counties of Louth, Cavan, Mona^han, Armagh, and Down,— form a narrow 

 stripe also along the coast of Antrim as far north as the Giant's Causeway, — 

 and, in the interior of Ireland, re-appear in the mountainous district of Tip- 

 perary. 



Soil. — The predominance of slaty rocks in this formation imparts to the soils 

 of the entire surface over which they extend one common clayey character. 

 They generally form elevated tracts of country, as in Wales, Cumoerland, 

 Scotland, and Ireland, where the rigours of the climate combine with the fre- 

 quent thinness and poverty of the soil to condemn extensive districts to worth- 



* Such a passage from one formation to another is exhibited in the diagrams inserted in 

 page 238. 



