Vol. 51.] RA.DI0LA.RIA.N ROCKS Df THE LOWER CULM! MEASURES. 627 



and radiolaria are abundant in them. The beds are very near the 

 boundary of the New Red Rocks ; but their relation to the dark 

 shales of this locality (? Culm or Devonian) is concealed. 



Respecting the thickness of the Radiolarian rocks generally, we 

 have not at present sufficient grounds on which to base an estimate. 

 The Rev. D. Williams 1 placed the maximum approximative thickness 

 of the series of his Codden Hill Grit, which included not only the 

 Codden Hill Radiolarian Beds, but also the dark shales and lime- 

 stones (Posidonomya-heds) down to the summit beds of the Upper 

 Devonian, at 1500 to 2000 feet, but a large proportion of this 

 would belong to the dark shales. In some of the sections of the 

 Radiolarian rocks exposed in the large quarries of North Devon 

 there is a vertical thickness of from 100 to nearly 200 feet shown. 

 In the southern area the exposures are less extensive, bat even here 

 the Carzantic quarry, in the Launceston district, has a thickness of 

 50 feet of radiolarian cherty rock without admixture of shales. It 

 is very probable that the thickness of the Radiolarian rock may vary 

 considerably in different areas, but it would perhaps not be rash to 

 assume that the Radiolarian Series may be some hundreds of feet in 

 thickness, not all indeed purely organic, but with intercalated beds 

 of fine shales. 



IV. Mode of Occurrence of the Radioi-ariaf Rocks. 



Both in the northern and southern areas the Radiolarian rocks 

 occur in evenly stratified beds, usually from 2 to 4 inches in 

 thickness, though they range from 1 to 9 inches, and in very 

 exceptional instances to a foot in thickness. The beds are inter- 

 sected in different directions by numerous fine and even joint-planes 

 which have the effect of dividing the rock up into comparatively 

 small rectangular or rhombohedral fragments with smooth, flat 

 surfaces. It is owing to this feature that these rocks are largely 

 quarried for road-metal, for which the harder beds are well adapted, 

 since in the process of quarrying they readily fall into these cuboidal 

 and rhombohedral fragments, which require very little additional 

 breaking to reduce them to suitable dimensions for this purpose. 

 In spite of these joints, however, the beds usually remain intact 

 until they are actually worked, and in the Barnstaple district and 

 in other places in North Devon quarries are not uncommon with 

 faces from 50 to 70 feet in height, showing a regular succession of 

 these thin, even beds extending uninterruptedly, generally at high 

 angles, from the summit to the floor of the quarry. 



1 ' Tabular Synopsis of the several Members of the Devonian System, etc' ; 

 Mr. Ussher, under the mistaken impression that this estimate referred only 

 to the Codden Hill cherts, etc., and did not include the Posidonomya-limeatonea 

 and shales, says that it could be obtained only by adding an unknown amount 

 of the overlying shales and sandstones to the Basement-beds, Proc. Som. 

 Archseol. & Nat. Hist. Soc. vol. xxxviii. (1892) p. 114. 



