CULM MEASURES. I99 



species are Lower Carboniferous types, although Phillipsia is known 

 in the Millstone Grit. (See p. 168.) 



The lower beds of the Culm-measures occasionally contain seams 

 of culm. Good sections of the beds are exposed at St. David's, 

 Exeter ; they extend to Holsworthy, and on the north side of the 

 Culm-trough they outcrop at Appledore and north of Bideford. 



Mr. T. M. Hall observes that the transition from the Upper 

 Devonian slates to the Lower Carboniferous shales is almost im- 

 perceptible, although the boundary is much concealed in North 

 Devon by detritus and superficial deposits. Thus immediately 

 north of Barnstaple the slates contain numerous Upper Devonian 

 (Pilton) fossils ; south of the town the slates, to all appearance 

 precisely similar, contain well-known Carboniferous species. 

 (See p. i2g.) 



The Culm-measures are exposed along the coast between Bos- 

 castle in Cornwall and Bideford in Devonshire, where in general 

 there is a splendid series of cliffs, those near Clovelly and Hartland 

 being especially remarkable, not only for their height, but also for 

 the manner in which the beds have been dislocated, crumpled up, 

 overturned, and contorted.^ North of Boscastle the beds are 

 traversed by large quartz-veins. 



It is generally considered that the Culm-measures were con- 

 solidated prior to the intrusion of the Dartmoor granite. In 

 the higher part of the Teign valley, near Lustleigh, and other 

 places bordering the granite of Dartmoor, the rocks are much 

 disturbed and metamorphosed, and there beds of indurated black 

 slate and grits with chert-beds occur.^ Some beds pass into lydian 

 stone : this is a very siliceous rock, containing carbon, which 

 gives it a grey or black colour, hence it is sometimes known as 

 Black Chert. Jasper also occurs in the so-called Carboniferous 

 rocks near Brent Tor: this is an impure and opaque form of 

 silica, coloured red, yellow or brown by oxides of iron. 



The Culm-measures in this area are not only broken by dykes of 

 " greenstone," but also by veins of quartz-porphyry (elvan). 



The boundary of the Culm-measures and Devonian rocks in 

 South Devon has not, however, been very definitely fixed in the 

 neighbourhood of Dartmoor, and Mr. F. Rutley informs me that it 

 is questionable whether the granite comes in contact with the Culm- 

 measures on the western side of this range. Near Tavistock the 

 beds are rich in metalliferous deposits. 



The Culm-measures, particularly between Barnstaple and South Molton, are noted 

 for the occurrence of Wavellite (hydrous phosphate of alumina). It was discovered 

 in a band of indurated shale at the latter locality about 1785. It exhibits its 

 radiating crystals on the broken transverse joints of the slaty rocks : hence the 

 term ' Wavellite schists.' The same mineral has also been met with in the Gower 

 Series of Glamorganshire. 



^ T. M. Hall, Geology of Devonshire (in White's History), 1878. See also J. 

 J. Conybeare, T. G. S. ii. plates 33 and 34. 



2 Dr. H. B. HoU, Q. J. xxiv. 411 ; G. W. Ormerod, Q. J. xv, 191, and Trans. 

 Devon Assoc. 1867. 



