Vol. 51.] RADIOLARIAN ROCKS IN THE LOWER CULM MEASURES. 661 



Launceston, Cornwall ; at Tavistock, and as far south as Painter's 

 Cross, near St. Mellion, Cornwall, where a small outlier is exposed. 

 On the east of Dartmoor they are shown at Ramshorn Down, near 

 Bovey Tracey, also near Chudleigh, and at Bishopsteignton. These 

 rocks, throughout, are characterized by radiolaria, occasionally 

 associated with sponge-spicules. These organisms in some instances 

 are thickly crowded in the beds, in other cases they are sparsely 

 scattered in a siliceous groundmass. Detrital materials are, as a 

 rale, absent in the Radiolarian Beds, but in some instances they 

 occur as microscopically minute fragments of mica and quartz. 

 Dark carbonaceous and ferrous materials, crystals of pyrites, and 

 rhombohedral crystals or negative crystals of carbonate of lime or 

 dolomite are also abundant in these rocks. 



The radiolaria occur both in the hard, cherty beds and in the soft, 

 friable shales ; they are not well preserved, and are now mostly in 

 the condition of siliceous casts. Forms belonging to 23 genera 

 have been recognized ; they are included in the following orders : — 

 Beloidea, Spheeroidea, Prunoidea, and Discoidea. In addition to 

 the radiolaria, a scanty but significant fauna of corals, trilobites, 

 brachiopods, and goniatites is present in some limited thin shaly 

 beds in the Barnstaple district. Nearly all the forms are diminu- 

 tive ; of the 23 species which have been determined, several are 

 only known elsewhere from the Lower Culm of Germany, others are 

 common to the Carboniferous Limestone of the British Isles and 

 Belgium. Prom the other divisions of the Lower Culm Measures 

 below the radiolarian rocks about 36 species are known, and 8 of 

 these occur in the Badiolarian Beds. 



No definite thickness can as yet be assigned to these beds. In 

 common with the other strata of the Lower Culm they have been 

 considerably disturbed, and continuous sections showing their rela- 

 tions to the other beds of the series are rarely shown. That they 

 are of no inconsiderable thickness is proved by sections in quarries 

 where a series of beds from 50 to 190 feet in vertical thickness are 

 exposed. 



Radiolarian deposits of a similar character to these fossil-beds 

 are now known only from great oceanic depths, and we are justified 

 in regarding these Culm radiolarian rocks as laid down in the deep 

 water of an open sea at some distance from a shore-line. Such a 

 conclusion must needs very considerably modify the current ideas 

 as to the character and origin of the Culm Measures in the South- 

 west of England, which have been regarded as a continuous series 

 mainly of detrital deposits of shallow-water origin. Thus we find 

 them described in recent text-books as a thick series of well-bedded 

 grits, sandstones, and occasionally thin limestones, which pass down 

 conformably into Upper Devonian strata and contain plants resem- 

 bling those in the Calciferous Sandstone Series in Scotland, as well 

 as animal-remains which point to a position low down in the 

 Carboniferous system. 



The facts now ascertained as to the real nature of the Lower Culm 

 Measures show that the series of rocks included under the general 

 Q. J. G. S. No. 204. 3 a 



