18   The "Dartmoor Dome"


is a special instance. It is somewhat of an outlier from the Moor, but it is pretty safe to claim it as an actual remnant of a carboniferous volcano, which in its prime must have been of considerable size. Mr. Clayden endeavours to show that the granite mass of Dartmoor is really the solidified upper part of the cooled lava reservoir from which the Carboniferous and Post-Carboniferous volcanoes of Devon were fed, and he asserts that it was at one time crowned not only by a great mass of culm strata, but also by extensive volcanic cones from which acid lavas were outpoured and from which explosive eruptions built up layers of volcanic ash. Simultaneously outflows of basic and other lavas were issuing from other vents, some derived from the basic pipes of the Teign Valley and other rocks which were dissolved away by the granite as it worked its way upwards. The fact that the elvans, or granite veins, closely related to the great granite mass, are more recent than the "great upheaval", seems to indicate that there must have been melted granite under the whole region at that period, and that its rise into something like its present position was determined by the particular conditions of the folding. The crystals of the granite show no signs of having been crushed or broken or exposed to any violent strain, as would have been the case had the granite been in position and cool before the upheaval. "We have a general accumulation of evidence... that Dartmoor is, in fact, the upper part of the cooled lava reservoir which fed our Devonshire volcanoes, and that it rose to its present place in consequence of the Post-Carboniferous earth movements, being then, and for some time, crowned or fringed by active, eruptive craters".

The unequal resisting power of various masses of the granite, and sometimes even of blocks of the same mass, has had a good deal to do in this matter of the tors and their "clitters", as well as influencing to no inconsiderable extent the nature of the river valley in the immediate neighbourhood of the Moor. The granite, although well able to resist abrasion when undecom-


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