I. Introduction. Considering that Dartmoor is the largest of English granite-masses, and displays almost every possible type of contact-metamorphism along its margins, its extensive literature contains surprisingly little detailed information regarding the features presented by the rocks at different localities round the Moor. Indeed, it may be said that, for an area so rich in points of interest, it has been much neglected by petrologists. Local workers are very few, and before the issue of the Geological Survey Memoir on Dartmoor there was no reliable summary of the various rock-types that were to be found. The petrographical contribution by Dr. Flett & Mr. Dewey has now supplied the want of a general account of the granite and its modifications, and of the rocks which constitute the metamorphic aureole round about it. Over much of the area covered by the Dartmoor sheet of the map, further work would now be chiefly of local interest: it happens, however, that the whole of the northern margin of Dartmoor is outside the area mapped, and that it presents some special features of its own. Moreover, the examination of the contact-zone in this northern area serves to suggest that detailed petrographical work on the distribution of the various rock-types may help materially in solving many questions as to the subdivision and succession of the Carboniferous strata. These have hitherto proved insoluble, owing to the lithological similarities and the general absence of fossils, as well as the numerous dislocations, in other parts of Devon. This paper