CULM MEASURES. I97 



The term Culm-measures was applied to the ' Carbonaceous ' 

 rocks of Devonshire on account of the workings for Culm near 

 Bideford and other places. The term ' Floriferous Slates ' was 

 used by the Rev. D. Williams, because plants are very generally 

 distributed, while culm is confined to a small area. 



Looked at in a large way, they consist of a series of black shales, 

 sometimes cleaved, greenish-grey sandstones and grits, occasional 

 beds of quartzose conglomerate, chert-beds, with bands of limestone 

 here and there. The shales are locally known as " Shillet," and 

 the sandstones as "Dunstone." 



The series occupies a trough between the Devonian rocks of 

 North Devon and West Somerset and those of South Devon and 

 Cornwall. The beds may therefore be said to occupy a basin or 

 synclinal, but they exhibit in almost every section faults and great 

 undulations and contortions, so that any estimate of their thickness 

 must be very vague. 



Some authorities have placed them, generally, on the horizon of 

 the Millstone Grit, but there seems to be good reason to include 

 with them representatives of at least a portion of the true Coal- 

 measures, and also of the Carboniferous Limestone, and Lower 

 Limestone Shales. (See p. i47-) 



The following general divisions may be made in the Series: — 



!/ Thick even-bedded grey grits, with slates 

 J n ^ :> ) '^''"^ shales. 



i.ower Coal-measures .'' j Sandstones, grits and shales, with beds 

 of culm. 

 Millstone Grit? Coddon Hill Beds (local). 



T r- . -i- T ( Shales, with thin grits, impersistent lime- 



Lower Carboniferous ? { ^^'^^^^ ^,^^ occasional beds of culm. 



The Culm-measures, above the Coddon Hill Beds, have been 

 described as containing ironstone and coal-measure plants, and 

 resembling the coal-deposits of Pembrokeshire. 



Some of the culm-beds of Devon have been considered to be 

 subordinate to the INIillstone Grit, although most of it overlies 

 the presumed equivalent of that formation, and is probably on the 

 same horizon as the culm of Pembrokeshire. 



Until the divisions of the Culm-measures are mapped out in 

 detail, we must refrain from committing ourselves to any definite 

 correlation, for Mr. W. A. E. Ussher, who is intimately acquainted 

 with the area, tells me it is quite possible that the Coddon beds 

 may underlie the limestones, or their representatives, near Venn, 

 Morebath, and Ashbrittle.' However, this uncertainty may arise 

 from flexures in the strata, which disturb their natural sequence. 



Among the fossils of the Culm-measures, the plant-remains 

 include Calainitcs Rocmeri, Sigillaria, Lepidodendron, Dadoxylon, 

 Pecopten's, Asterophyllites, etc." Anthracosia and Calacanthus have 



* Proc. Somerset Arch, and Nat. Hist. Soc. vol. xxv. 1 881. 

 2 See T. M. Hall, Trans. Devon Assoc, vii. 367, and Geology of Devonshire 

 (in White's History), 187S ; also R. Kidston, G. Mag. 1884, p. 534 (footnote). 



