The object of this investigation was to examine the well-known Hestercombe “syenite” in greater detail than had been done before, to attempt to trace its extent, and if possible, to determine its age.

One of the first features that will probably strike a geological observer in Jamaica is the extent and thickness of the great covering series of White Limestones. This series, which as Sawkins wrote in 1865, cannot be less than 2,000 feet in thickness, impresses one with the evident magnitude of the duration of the Tertiary period; especially when one reflects that it represents only a portion of the Tertiary, probably inclusively from the Upper Eocene to the Lower Miocene. Equally formidable though less conspicuous thicknesses of early Tertiary beds which underlie the White Limestones, namely, the Yellow Limestones and the Richmond beds or Carbonaceous Shale, represent the middle and basal Eocene.

The area studied includes the majority of the main granite masses north of the belt of Ordovician and Silurian rocks running eastwards from Quimper to around Vitré and Laval. The heavy minerals of the Channel Islands granites have already been described by the writer (1). In the present paper the work is extended to the northern half of Finistère, and the greater part of the Côtes-du-Nord, Ille-et-Vilaine, and Manche. It is confined for the most part to the granites, and comparisons are made with those of the Channel Islands in an endeavour to obtain more definite evidence as to the age of the Jersey “Newer” Granites.