The oil wells of Cuba so far have not produced anything like the enormous quantities that issue from the wells in the United States and Mexico, but the results are encouraging, especially since the explorations so far have been confined to a very moderate depth, seldom exceeding 1500 feet. It is quite probable that wells in this section will be ultimately drilled to a depth of at least 4,000 feet. Petroleum, as we know, is found in many different kinds of geological formations. In Pennsylvania we meet crude oil in the Devonic and carboniferous strata; in Canada in the Silurian; in the State of Colorado in the cretaceous; in Virginia in the bituminous coal lands; in South Carolina in the Triassic; in Venezuela it occurs in mica formations; while in the Caucasus again it is in the cretaceous. No fixed rule therefore can be said to designate or control the geological formation that may yield oil. All of the petroleum found in Cuba, so far, seems to have its origin in cretaceous formations, corresponding probably to the Secondary. A somewhat significant fact is that petroleum in this Island seems to be invariably associated with igneous rocks. So far all of it, or at least all in wells worthy of consideration, seems to come from deposits that lie along the lines of contact between the serpentines and various strata of sedimentary rocks. Up to the present, wells that have been drilled in sedimentary strata, at any considerable distance from the intrusion of serpentine rocks, have produced no results. E. de Goyler has reached the conclusion that the oils found below the serpentine, or at points of contact between serpentine and sedimentary rocks, had their origin in Jurassic limestone. Rocks of this period form a large part of the Organ Mountains of Pinar del Rio, and the above quoted authority is confident that the asphalt and petroleum fields found in the immediate vicinity of serpentine thrusts during volcanic action are all filtrations from deposits far below the surface. This view seems to agree with results of observation made in the neighborhood of the Bacuranao oil fields, where the drills have usually penetrated a considerable depth of serpentine rock before meeting the petroleum-bearing strata of sand and limestone. Frederick C. Clapp, in his study of the structural classification of fields of petroleum and natural gas, read before the Geological Society of America, stated that in Cuba there are undoubtedly deposits which he designates as coming from a subdivision of sedimentary strata, with masses of lacolites, an unusual form of deposit, met in the Furbero Petroleum fields of Mexico, where oil bearing strata lie both above and below the lacolite.