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THE WONDERS OF GEOLOGY. Lect. VIII. 



Ireland and Wales, and the Ocrynian chain of Cornwall, 

 were above the waters. 



3. Subdivisions of the Devonian System. — The 

 uppermost beds of this system, for the most part, dip con- 

 formably beneath the mountain limestone, or other members 

 of the Carboniferous, and the lowermost pass into strata 

 that belong to the upper series of the Silurian. For the 

 convenience of study, the deposits comprised in this forma- 

 tion are subdivided into three groups : — 



I. Quartzose Conglomerates and Sandstones and Marls. The 

 sandstones are often either of a deep chocolate red, or greenish 

 colour. The marls partake of the same tints, but are frequently 

 mottled with blotches of red and green. Fishes of the genus 

 Holoptychius. 

 II. Cornstone and Marls. Irregular bands of concretionary lime- 

 stone, provincially termed Cornstone ; with intercalations of 

 marl; the same colours prevailing as in the upper group. 

 Abound in remains of Cephalaspis and other fishes. 

 III. Tilestones. Finely laminated quartzose or micaceous sandstones, 

 termed tilestones, from the facility with which they are sepa- 

 rated into thin slabs or tiles. Contain shells, and fishes of the 

 genus Dipterus, &c. 



The total thickness of this system in Herefordshire and 

 South Wales, is estimated by Sir R. Murchison at 10,000 

 feet. Of its extent, in this part of Britain, the following 

 remarks of the same eminent geologist will give a clear 

 idea. " Occupying the largest portion of Herefordshire, 

 and the adjacent districts of Worcestershire and Shropshire, 

 it spreads over wide tracts of Monmouthshire, surrounding 

 the coal-field of the Forest of Dean ; and forming a girdle 

 round the great South Welsh coal basin, it constitutes in 

 Brecknockshire the loftiest mountains of South Britain. 

 The enormous thickness of the red stratified deposits in- 

 cluded between the coal-measures and the Silurian rocks, 

 will at once be comprehended by any observer who places 

 himself on the eastern slopes of the latter on the Welsh 



