Vol.51.] RADIOLARIAN ROCKS IN THE LOWER CCTLM MEASURES. 615 



III. Distribution op the Radiolarian (Codden Hill) Beds. 



The references given above show that the general petrological 

 characters of the Codden Hill Beds are sufficiently distinct from 

 those of the rocks above and below them in the same series to have 

 caused them to be placed as a separate subdivision by observers who 

 were unaware of their organic characters. But we do not consider 

 that all the rocks which have been referred to this division by 

 Mr. Ussher and other geologists properly belong to it, and we wish 

 further to note that in one particular feature the description given 

 by nearly all previous writers of the character of the Codden Hill 

 Beds materially differs from what we have observed. Thus, it is 

 stated 'by Williams that they are mainly of 'black-layered, fine- 

 grained grit ' ; by Phillips that the chert is associated with soft, 

 white, arenaceous layers ; Hall conjoins grit and chert ; Sir R. 

 Murchison writes of the Goniatite-grits of Codden Hill ; and 

 Mr. Ussher also includes fine shaly grits with the cherts and shales 

 of the typical beds. But neither at Codden Hill itself nor at the 

 outcrop of these beds in any part of the Culm area have we met 

 with any description of grit, or shaly grit, or arenaceous shales, asso- 

 ciated with the cherts and shales containing radiolaria. There are, 

 it is true, some beds which on ordinary superficial examination bear 

 considerable resemblance to fine grits, but microscopic sections of 

 these show that they are filled with radiolaria and that their sup- 

 posed gritty structure arises from these organisms, which are more 

 resistant than the soft siliceous shales in which they are embedded. 

 The Radiolarian or Codden Hill Beds occur along a comparatively 

 narrow zone of country a short distance within the boundary-line 

 of the Culm and the Devonian, which runs in a generally E.S.E. 

 and W.N.W. direction both in North and South Devon. We propose 

 to refer first to the series in North Devon, beginning with the 

 exposures round Barnstaple and then going eastwards to Dulverton, 

 Ashbrittle, and Holcombe Bogus. On the southern boundary of 

 the Culm Measures we commence on the coast near Boscastle and 

 continue eastwards to the district beyond Launceston, and then to 

 Tavistock. On the east of Dartmoor we refer to exposures at 

 Bamshorn Down near Bovey Tracey, at Chudleigh, and at Bishops- 

 teignton near Teignmouth. 



(a) Barnstaple District, North Devon. 



Codden Hill itself may be taken as a convenient starting-point 

 from which to follow the outcrop of the Radiolarian rocks in 

 North Devon and West Somerset. The Hill is situated about 

 3 miles south-east of Barnstaple, following the high road on the 

 east of the Taw River through the village of Bishop's Tawton. 

 It is an elongate, hog-backed ridge, about 1| mile in length, 

 running nearly east and west, and between 500 and 600 feet in 

 height. The entire ridge consists of siliceous radiolarian rocks, 

 which are well shown in several quarries on the western and 

 southern slopes. At the western end the principal exposure is known 



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