It is not altogether pleasant to deal with a subject in regard to which the domain of our ignorance is so broad; but if we are optimists we may be comforted by the reflection that the geologists of this generation, at least, will have no occasion like Alexander, to lament a dearth of worlds to conquer.

Annual Report of the Board of Regents of the Smithsonian Institution, 1895, Continental Problems of Geology (p. 173)