The name of this rock is derived from the locality where it occurs, viz. from Mount Foya, in the south of Portugal. It is cursorily referred to as granite in Charles Bonnet's description of the Province of Algarve; and as early as 1861 specimens of it were submitted to Professor Blum, of Heidelberg, who was the first to examine its composition. This distinguished petrologist at once recognized it as a true nepheline-bearing syenite; and so struck was he with the close resemblance it bears, not only to certain Norwegian syenites (often erroneously termed zirconia-bearing syenites), but also to a syenite occurring in one of the Cape-Verd islands, that he united them all into a separate group called Foyaites. Of these, however, the Foyaite proper is so rarely met with that, to the large majority of petrologists, it is almost terra incognita; nor is the locality itself easily accessible to geological research. I have been enabled to examine under the microscope more than 40 sections of this rock representing different parts of the locality; and so interesting and varied is its composition, that it may not be inopportune to give a description of it, which I shall preface by a rapid sketch of the leading features of the Foya district, as illustrated by the annexed plan and section (Plate I.).