Vol. 51.] EADIOLAKIAN ROCKS IN THE LOWER CULM MEA8TJRES. 625 



dark or bluish grey, and traversed in all directions by veins of 

 quartz. The rock is filled with radiolaria, some even showing 

 traces of structure. Another small patch of similar chert is seen in 

 the foundation of some cottages on the east side of Vinegar Hill, 

 south of Painter's Cross. 



The district north of Stoke Climsland is also stated by Dr. Holl ' 

 to show chert, but none could be found there. On the high ground 

 just south of this village, however, there are some beds, from 

 1 to 3 inches in thickness, of partially decayed bluish chert with 

 shales dipping S., but no organisms could be recognized in it. 



(h) Ilamshorn Down, near Bovey Tracey, S.E. Devon. 



Passing now to the Culm Measures east and south-east of Dart- 

 moor, in the narrow irregular area between the granite and the 

 Devonian rocks, there is an important outcrop of cherty rocks on 

 Ilamshorn Down, a prominent rounded ridge about 3 miles S.S.W. 

 of Bovey Tracey, to which our attention was called by Mr. Ussher. 

 Two quarries are shown by the side of the road which skirts the 

 western end of the Down, one about | mile north of the other. 

 The rocks in both quarries are very similar ; they mainly consist of 

 very hard, dark, siliceous rocks and a few bands of lighter shaly 

 material, resembling very closely in character those of Codden Hill 

 in North Devon and of the Launceston area in Cornwall. Radio- 

 laria are very abundant in the dark beds and also in some bands of 

 a bluish or brownish tint ; sponge-spicules are also associated with 

 them. In the northern quarry a vertical thickness of about 25 feet 

 is shown ; the dip is 38°-40° S.E. In this thickness there is but 

 one bed, about 6 inches in thickness, of the light shaly rock ; the 

 rest is either dark or bluish brittle rock, or of a platy character. In 

 the southern quarry the cherty beds have been locally crumpled 

 and pushed over one another, and Mr. Ussher has referred to and 

 figured this as an instance of false bedding, 2 which of course would 

 indicate the action of currents during their deposition. But, so far 

 as we could observe, the present disturbed condition of the beds 

 results from mechanical forces acting long after their formation. 



Mr. Ussher 3 considers that the chert-beds of Ilamshorn Down 

 and its vicinity rest on the Upper Devonian and apparently below 

 the shaly arenaceous beds and Posidonomi/a-b&nds, and thus on a 

 different horizon from the Badiolarian Beds of Codden Hill and 

 North Devon generally, and from those of the Launceston area, 

 which, so far as present evidence goes, are above the Posidonomya- 

 beds. The actual contact of these beds with the Devonian slates 

 has not, however, as yet been seen. 4 It is further stated that the 

 chert-beds seem to pass beneath sandstones and shales with traces 

 of plants, but here again the succession has not yet been definitely 

 ascertained. 



1 Quart. Journ. Geol. Soc. vol. xxiv. (1868) p. 404. 



2 Proc. Som. Archseol. & Nat. Hist. Soc. vol. xxxviii. (1892) p. 134, fig. 2. 



3 Ibid. pp. 134, 135. 4 Ibid. pp. 134, 144. 



