In the formation of a country three more or less distinct geological operations are concerned:—a. Rock formation; b. Earth movements; c. Denudation. In the case of our own country, to which the present paper more especially refers, these three operations have succeeded each other again and again; so that a very complicated structure is the result. The earth movements and denudations of one period have been interfered with by those of subsequent periods, and any attempt to trace the exact steps by which the evolution of the British Isles has been effected is therefore attended with very great difficulty.