It is, of course, impossible to know the geological history of a country until its paleontology has been thoroughly studied and investigated. Where formations of different geological ages are lithologically so similar as to be often indistinguishable, the only method of differentiating them is by their fossils. Some paleontological work has been done in Cuba, but the specimens collected were not accompanied by the necessary data. In the present imperfect state of our knowledge of the stratigraphy and areal geology of the island, it would be hazardous to attempt to indicate the times at which the various levels were developed, or to designate the periods during which they remained above the level of the sea. To do this would require a detailed knowledge of nearly all the various phases of its geology. The oldest rocks in Cuba, with the possible exception of the schistose limestones of Trinidad, are composed of granites and serpentines. The relative age of these rocks, to the central mass of limestones in the province of Pinar del Rio, has not been determined, but we do know that the oldest igneous rocks were themselves folded, faulted and subjected to other processes of metamorphism, and that subsequent to the changes to which they were subjected, the entire region was uplifted and deeply eroded before the cretaceous sedimentation began. No data are available for determining the geologic period at which the pre-cretaceous erosion began, but the region has doubtless been standing above the waters of the ocean for a very long interval, since the amount of rock carried away has been manifestly great. The surface upon which the cretaceous sediments were deposited, appears to have been reduced by erosion to a very low relief, so that the land was a featureless plain when the cretaceous subsidence began. The time interval required for the accomplishment of this erosion must have been very long, since when it began the region was undoubtedly mountainous. The complex character and disturbed altitude of the pre-cretaceous rocks, the granites, diorites and other granular rocks which appear on the surface because of this erosion, were originally formed deep within the crust of the earth, and therefore furnish a reason for believing that this period of erosion was exceedingly long. It has been suggested that during the Jurassic times, the southeastern coast of the United States was connected by a long isthmus, following the line of the Antilles, to the northeastern coast of South America. The data presented would seem to indicate that at least the eastern half of Cuba stood high above the level during this period of the earth's history, and although data concerning the western half are less definite, it too was probably composed of high land masses.