Half a mile east of Cayo Flamenco we passed close to two rocks on which the waves break furiously. They are the Piedras de Diego Perez (latitude 21 degrees 58 minutes 10 seconds.) The temperature of the sea at its surface lowers at this point to 22.6 degrees centigrade, the depth of the water being only about one fathom. In the evening we went on shore at Cayo de Piedras; two rocks connected together by breakers and lying in the direction of north-north-west to south-south-east. On these rocks which form the eastern extremity of the Jardinillos many vessels are lost, and they are almost destitute of shrubs because shipwrecked crews cut them to make fire-signals. The Cayo de Piedras is extremely precipitous on the side near the sea; and towards the middle there is a small basin of fresh water. We found a block of madrepore in the rock, measuring upwards of three cubic feet. Doubtless this limestone formation, which at a distance resembles Jura limestone, is a fragmentary rock. It would be well if this chain of cayos which surrounds the island of Cuba were examined by geologists with the view of determining what may be attributed to the animals which still work at the bottom of the sea, and what belongs to the real tertiary formations, the age of which may be traced back to the date of the coarse limestone abounding in remains of lithophite coral. In general, that which rises above the waters is only breccia, or aggregate of madreporic fragments cemented by carbonate of lime, broken shells, and sand. It is important to examine, in each of the cayos, on what this breccia reposes; whether it covers edifices of mollusca still living, or those secondary and tertiary rocks, which judging from the remains of coral they contain, seem to be the product of our days. The gypsum of the cayos opposite San Juan de los Remedios, on the northern coast of the island of Cuba, merits great attention. Its age is doubtless more remote than historic times, and no geologist will believe that it is the work of the mollusca of our seas.