Physical Structure and older sti^atified Deposits of Devonshire. 671 



These shales, in the ascending sections, frequently become semi-indurated ; and in that condition 

 are generally thin-bedded, and the separate beds are divided into innumerable prismatic masses by 

 small transverse joints. In the softer varieties these masses are generally rhombohedral. Again, the 

 indurated shales pass into rather thin-bedded, hard, siliceous bands, each of which becomes sepa- 

 rated into cuboidal divisions ; and the surfaces of the beds have what the workmen call a diced form, 

 resulting from the intersection of parallel cross joints. This structure seems to be derived, in a great 

 measure, from a tendency to a concretionary structure. These harder bands are often of a dark 

 colour, and the partings are coated with earthy carbon, which stains the fingers. It is among the 

 little transverse cracks and joints of the harder beds that a white earthy mineral greatly abounds. 

 The white divisional planes are due sometimes to carbonate of lime ; but through a considerable tract 

 running near the base of the culm series (between Barnstaple and South Moulton) the white mineral 

 is chiefly composed of earthy Wavellite ; and occasionally the broken transverse joints exhibit the ra- 

 diating crystals of that mineral in great perfection*. Still larger parallel joints, affecting the whole 

 series, also occur ; but these we are not at present describing. The same hard bands vary, however, 

 greatly in colour, where the carbon is not present ; and in several places they pass into a light 

 gray, siliceous, brittle, jointed flagstone : and again, in some remarkable instances, they become 

 almost white and porcellaneous, resembling a kind of china stone. Again the siliceous beds pass into 

 a regular thick-bedded sandstone, resembling a coarse gray-wacke, or a grit of the coal measures : 

 these latter varieties are, however, on the northern boundary (from which our present descrip- 

 tions are chiefly taken) to be considered as the exception rather than the rule, and as subordinate 

 to the carbonaceous shales and flagstones. 



The beds forming the base of the culm series are somewhat differently developed on the south- 

 ern boundary, in their range from the Cornish coast towards Dartmoor. We first meet with a 

 highly carbonaceous and pyritous shale, dipping at a great angle from the chloritic and fossiliferous 

 slate group of Tintagel. These alternate with coarse quartzose bands, sometimes thickly bedded, 

 and resembling coarse beds of gray-wacke. The whole system is much intersected by joints, and 

 traversed by large quartz veins f; and has so much the look of an old rock, that it has always 

 been regarded as a regular part of the old slate series of Cornwall. The mineral character, how- 

 ever, as well as the superposition, is sufficiently distinct ; and in several places (for example, on 

 the north side of Boscastle) the coarser bands resemble a coal grit, are stained with carbon, and 

 contain many very thin laminae of that substance +. Some of the shales are also calcareous, and 

 contain masses of a variety of rottenstone, like that in the limestone shales of Derbyshire; but 

 none of them contain any fossils resembling those of the older Tintagel slates. Among the higher 

 beds of the series, we have a great development of black, semi-indurated shales ; but no true 

 bands of black limestone ; and the whole group is violently contorted. 



Following this bottom group to the east, along the line of strike, we still find the dark lower 

 shales more or less carbonaceous, and with traces of bright anthracitic laminae ; and here and 

 there (though rarely) with obscure traces of vegetable fossils §. And along the same line, further 



* The same mineral occurs among the indurated coal shales of South Wales. 



+ These veins are sometimes several feet in thickness. They usually run parallel to the beds 

 for some way, and then come to an edge, or break off into small strings, ramifying through the 

 neighbouring beds in all directions. 



X These beds, or some resembling them near Boscastle, were first described by Dr. Boase, in 

 the Transactions of the Geological Society of Cornwall, vol. iv. p. 426. 



§ The descriptions of the preceding paragraph are taken from the lower culm beds near Bos- 



