1868.] IIOLL SOUTH DEVON AND EAST CORNWALL. 441 



trend up to it, for ob^dous reasons ; and it varies also with the rela- 

 tive fusibility of the rock, the volcanic rocks being the first to exhibit 

 any alteration, which they do by becoming crystalline and altered 

 in the arrangement of their elements, as near Petertavy and on the 

 east of Dartmoor. Among the Culm-measures the resultant meta- 

 morphic rock is commonly either a minutely crystallized black schorl- 

 rock of uniform texture, or a mixture of this with semivitrified grit 

 in alternating layers, or a uniform mixture of small schorl- crystals 

 and quartz -grains. Some of the rocks, however, are rendered mica- 

 ceous ; and among the older rocks this result is the more common 

 effect ; but occasionally crystals of chiastolite, more or less perfectly 

 developed, appear in the slates, as on the north of Ivy Bridge. None 

 of these altered rocks, however, exhibit a high degree of metamor- 

 phism, but the contrary ; and we pass abruptly from these slightly 

 altered rocks to highly crystallized coarse-grained granite, without 

 any intervening rocks which would indicate a gradation from the un- 

 altered Carbonaceous and Devonian rocks into the granite. The 

 truly igneous character of these granitic masses is therefore as clearly 

 shown as any geological phenomenon can be ; and around the margin 

 o£ the moor the granite has thrown out veins, both large and small, 

 into the adjacent rocks. Whether the elvans, which atBlisland and 

 St. Neots also appear to have emanated from the molten mass, 

 really did so or not, we need not stop to inquire. 



YI. General Remarks. 



It follows from what has been stated that neither the highest nor 

 the lowest portion of the Devonian system, as seen in North Devon, 

 occurs on the south side of the Culm-measures, and that the slate 

 rocks which pass under the Carbonaceous series on the north are not 

 the same as those that rise from beneath them on the south. Ther'^ 

 is, no doubt, a good deal of similarity between them, as there is be- 

 tween much of the slaty rocks of Devonshire ; and this, together with 

 the fact that both are locally fossiliferous and contain some organic re- 

 mains in common, has led to the inference that the opposite sides of the 

 trough are symmetrical. So far as the Culm-measures are concerned, 

 this appears to be the case ; and although the local admixture of vol- 

 canic material may have somewhat augmented the thickness of these 

 beds in the south, the small patches of limestone at a certain distance 

 from the base of the series, both in the north and in the south, ap- 

 pear to indicate a well-marked horizon on which there existed a 

 change of conditions ; and hence the subdivision of the Carbonaceous 

 system by Sedgwick and Murchison into a lower and an upper series 

 is a natural one *. But in the south there is complete unconform- 

 ability between the base of the Culm-measures and the underlying 

 Devonian rocks. If we follow the southern range of Culm-measure 

 limestones from Launceston eastward, round the margin of the Dart- 

 moor granite, by Bridestow to Drewsteignton, and thence to the 

 north of Dunsford, we find that the interbedded volcanic rocks be- 

 tween Dunsford and Chudleigh correspond in their relative position, 



* L. c. p. 670. 



