This paper forms part of a communication recently made to the Geologists Association, with some additional matter relative to certain criticisms passed upon Mr. Dunn's hypothesis as to the origin of the diamonds.

It is taken for granted on this occasion that the leading geological facts are known, I will merely, therefore, draw attention to a generalised section of Kimberley Mine, where one side represents the wall or reef bounding the pipe itself, and is thus constituted:—

The great pyrites masses of the Sierra Morena, of which those worked at Rio Tinto are by far the most extensive, may be briefly described as contact deposits, which differ from such as are ordinarily met with only in size; and from ordinary fissure veins only in the circumstance that the cavities in which the minerals occur are bounded by dissimilar rocks.

The granite of the Ord forms one, and that probably the smallest of a number of masses which constitute the border-land between Caithness and Sutherland. The features of this granite vary considerably at different points. It will, however, be afterwards shown that the rock is possibly continuous from north to south,--that is, from Sandsidc Bay, on the Pentland Firth, to Navidale, near Helmsdale.