The numerous specimens illustrating the rock formations of the Isle of Rodriguez, collected by Mr. I. Bayley Balfour from different localities, need only a cursory inspection to attest the volcanic character of the whole mass of the island. Rodriguez, in fact, consists of Doleritic lavas that appear to have been poured out at a considerable number of volcanic orifices at successive periods. It would be difficult, without more minute description of the physical geography of the island than is accessible, to assign any precise date or duration to these volcanic eruptions, or to trace with any certainty the degree to which and the mode in which subse­quent denudation has helped in giving the island its present remarkable aspect.