The area which it is the purpose of this paper to describe is of interest to petrologists because of the occurrence within its borders of the rare ultrabasic rock cumberlandite, of a considerable mass of riebeckite granite and associated porphyries, and of the remarkable quartz deposit known locally as Diamond Hill. As will appear beyond, all three of these formations, as well as the general geology of the region, have been made the subject of more or less study by workers in this field. A desire to ascertain more fully the relations and characteristics of the riebeckite rocks was the particular motive that led the present writers to take up the study of the region, and while this phase of the investigation was the main object, it has been thought well to extend somewhat the scope of the paper, so as to include a description of the surrounding formations . . .