010 BR. G. J. HINBE AND MR. HOWARD FOX ON [Nov. 1895, 



Mr. W. A. E. Ussher, who has spent many years in investigating 

 this formation on behalf of the Geological Survey, have been 

 principally in the direction of detailed filling up of the outlines 

 laid down by the older geologists. 



Stated briefly, the Devon Culm Measures have hitherto been 

 described as consisting of a lower or basal series of dark argillaceous 

 shales with impersistent intercalated beds of dark limestone, 

 which conformably succeed the fossiliferous shales and slates of the 

 Upper Devonian. Next above this basal series there is a succession 

 of what have been termed fine shaly grits and cherty mudstones, 

 known as the ' Codden Hill Beds.' Overlying these latter 

 there is a further series of sandstones, grits, and shales of 

 o-reat thickness, with occasional thin layers of Culm, which form 

 the higher beds. The entire series of the Culm Measures, as first 

 shown by Sedgwick and Murchison, occupies a synclinal trough 

 between the Devonian rocks of North Devon and West Somerset 

 and those of Mid-Devon or South Devon and Cornwall. The 

 rocks throughout have been highly folded and contorted as well as 

 faulted, and in the southern area bands of tuff and diabase are not 

 infrequent in them. A somewhat scanty fauna is found in the 

 limestones and shales of the basal series, consisting of a few species 

 of Goniatites, Posidonomya, GJionetes, Spirifer, and Fhillipsia, 

 while from the next succeeding Codden Hill Beds only about four 

 or five species of Goniatites and brachiopoda have hitherto been 

 known. From the higher portion of the Culm Measures a number 

 of plants and two species of fishes have been determined. It will 

 thus be seen that if we except the thin lenticular beds of limestone 

 in the basal series and the occasional layers of Culm in the higher 

 series, the Culm Measures are at present regarded as consisting of 

 a sequence of detrital or clastic mechanical deposits, which might 

 have been laid down in comparatively shallow waters near a land 

 area. 



We have ascertained, however, that the division next above the 

 basal limestones and shales, known as the Codden Hill Beds, is 

 essentially of organic origin, and that it is filled to a great extent 

 with the remains of radiolaria, thus probably forming one of the 

 thickest deposits of these microscopic organisms hitherto known 

 in the geological series. We have made an examination of nume- 

 rous exposures of the rocks of this horizon at various localities 

 along their line of outcrop in North Devon and West Somerset 

 from Templeton, south-west of Barnstaple, to Ashbrittle, near 

 Wellington, a distance in a direct line of about 31 miles ; also on 

 the southern outcrop from near Boscastle on the Cornish coast to 

 Tavistock for a distance of about 26 miles, as well as on the east 

 and south-east of Dartmoor, near Bovey Tracey and Chudleigh : 

 with the result of finding that, throughout, the beds of this horizon 

 are characterized by the presence of radiolaria. We propose in 

 the present paper to give a sketch of the distribution, the principal 

 features and microscopic characters of these Codden Hill radiolarian 

 rocks in the different localities, and to describe the radiolaria in 



