A HISTORY OF DEVONSHIRE 



the southern outcrop of these rocks, render the relations of the Lower 

 Culm in the latter area much more complex than in the former. 



Lower Culm. — In both areas where the succession is complete the 

 upper beds consist of shales with hard mudstones with limestone bands. 

 In the northern outcrop this subdivision sometimes mainly consists of 

 shales with mudstones ; sometimes of shales, mudstones and limestones ; 

 sometimes, as in the Westleigh quarries, chiefly of limestone, in which 

 chert from the segregation of silex is locally met with, as well as dolo- 

 mitic rock. In the southern outcrop, from the Tamar to Doddiscombs- 

 leigh, this horizon is distinguished by blackish shales with hard even 

 mudstones and occasional developments of Hmestone ; it makes a plicated 

 sinuous junction with the overlying grits and shales which carries it 

 southward round Dartmoor. It is recognizable in the inliers in the 

 Middle Culm rocks north of Kingsteignton and near Bickington. Posi- 

 donomya becheri is the characteristic fossil of these beds but Goniatites 

 spiralis is even more plentiful in the Chudleigh district, especially near 

 Waddon Barton. 



Below the Posidonomya beds come the Coddon Hill chert beds ; these 

 and all the Lower Culm cherts of this horizon collected by Fox and 

 examined by Dr. Hinde yielded Radiolaria. On the northern outcrop 

 these beds are generally parti -coloured, grey, pale buff and white ; on the 

 southern outcrop they are pale grey, dark grey, or black. Near Bick- 

 ington they form the base of the Culm, and occur as outliers on the 

 Upper Devonian near Tamerton Folliott. 



The thickness of the chert beds is variable, and there is reason to 

 think that this horizon is locally represented by bands of chert in dark 

 shales and mudstones. It is so plicated that it is difficult to believe that 

 its maximum development attains to loo feet. 



The basement beds of the Culm consist of dark shales with occas- 

 ional seams of chert or mudstone in the northern outcrop, and with hard 

 even bedded, sometimes pale banded, mudstone in the upper part in south 

 Devon, where present. Contemporaneous volcanic rocks seem to be 

 associated with these beds in the Tavistock area. In the Ashton and 

 Trusham district contemporaneous tuffs, etc., come in, in, or just above, 

 the chert beds. 



Middle Culm. — The Middle Culm Measures are a variable series ; 

 they contain interbedded (thin or thick) grits and shales ; irregularly 

 associated hard, even and irregular, developments of grit in shales ; even- 

 bedded sandstones and marly-splitting sandy shales ; coarse and fine grey- 

 wacke locally conglomeratic, associated with dark uneven shale in variable 

 proportion. 



Their lower beds vary considerably in different parts of the area. 

 Between Crediton and Chudleigh they consist of hard, thin, even bedded 

 grits intercalated in splintery shales ; in the northern outcrop, of inter- 

 calations of even bedded grits (with anthracite seams near Bideford, 

 Alverdiscot, etc.), sandstones and shales. On the south of Chudleigh 

 the sandstone and greywacke type prevails. In Ugbrooke Park and 



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