The author has found it impossible to compress into two volumes, according to his original plan, the wide range of subjects which must be discussed, in order fully to explain his views respecting the causes of geological phenomena. As it will, therefore, be necessary to extend the “ Principles of Geology” to three volumes, he prefers the publication of the present part without delay, because it brings to a close one distinct branch of the inquiry, the study of which will be found absolutely essential to the understanding of the theories hereafter to be proposed. Considerable progress has already been made in the remainder of the work, which will shortly be laid before the public.