Vol.51.] RADIOLAEIAtf EOCKS IN THE LOWER CULM MEASTTEES. 663 



rocks. These are of much less thickness than the equivalent 

 Carboniferous Limestones, but this may arise from the fact that 

 the siliceous organic deposits are of far slower accumulation than 

 calcareous beds, and they may well have occupied in their formation 

 an interval of time as great as that taken by the formation of the 

 limestones. 



It is worthy of note that the deep-sea character of the Lower 

 Culm of Germany, which corresponds with our Lower Culm Mea- 

 sures, had been maintained by Dr. Holzapfel, 1 even before the 

 discovery of radiolaria in the beds of Kieselschiefer furnished such 

 strong evidence in support of this view. 



Much field-work has to be done before a satisfactory knowledge 

 of the ' British Culm Measures ' can be obtained, and the present 

 paper only professes to be an introduction to the study of the one 

 division of the Radiolarian Beds. The Upper Culm Measures, which 

 in our opinion should include the beds above the Radiolarian rocks 

 (with perhaps the exception of the shales with fishes and goniatites 

 at Instow), have only incidentally been considered, and the question 

 of their relative age can be determined only by their plant-remains. 

 We have, further, not referred to the igneous rocks which in Central 

 Devon and Cornwall are of not infrequent occurrence in the Lower 

 Culm Measures. 



Postsceipt. 



[Since this paper was read, one of us has examined the Lower 

 Culm area to the north of the granite of Dartmoor, between Bride- 

 stow and Drewsteignton, in order to ascertain if the Radiolarian 

 rocks could be also recognized in this district. The rock-exposures 

 are mainly limited to the quarries, now for the most part disused, 

 which have been worked for the Culm limestones. The following 

 is a brief notice of the different exposures, beginning at Bridestow 

 and from thence to the north-east : — 



Bridestow. — In the large disused limestone-quarry about half 

 a mile S.W. of the village there are angular masses of chert 

 remaining on the refuse-heaps, and at the S.W. end of the quarry 

 chert is shown in position associated with grey shale. The chert is 

 dark, and traversed throughout with thin quartz-veins ; microscopic 

 sections show in it traces of radiolarian casts and also numerous 

 rhombohedral crystals. 



Sourton. — In the limestone-quarry about half a mile east of 

 Bridestow village, a clear vertical section of more than 100 yards 

 in length is exposed. The beds at the top consist of chert inter- 

 bedded with dark siliceous shales : below these are impure limestones 

 and shales, and, near the base of the section, dark limestones and 

 shales. The prevailing dip is N.N/W. ; the upper beds are much 

 folded. The chert of the uppermost beds is partly dark and partly 

 of a bluish tint ; a microscopic section of the dark variety showed 



1 Palaeont. Abhancll. n. s. vol. i. (1889) p. 6 ; see also ' Text-book of Comp. 

 Geol.' Kayser-Lake, 1893, p. 126. 



3a2 



