In the course of a geological reconnaissance of Andros Island, in the Bahamas, it was found that the lower forms of plant life, especially the Blue-green Algæ, play an important part in the process of sedimentation. In addition to those forms which actively contribute calcium carbonate to the sediment, there are other species which function primarily as sediment binders, without necessarily precipitating any lime themselves. Such sediment-binding algæ usually impart characteristic structures to the medium in which they grow; and in the interior of Andros, where such deposits are now accumulating over large areas, structures are being produced which are reminiscent of those found in some of the great limestone formations of the Lower Palaeozoic and Upper Precambrian. In view of this, and of the supposed algal origin of certain of these limestone structures, it is felt that a detailed description of the Bahaman sedi- merits will provide an example of a modem Cyanophyceous deposit, which, may prove useful for comparison with older limestones of similar structure.