In ordinary circumstances crystal-angles are measured by means of light reflected from the crystal-faces. In the partial or complete absence of faces the determination of axial ratios and angles may be impossible. In a recent investigation of certain derivatives of glyoxaline, an example of this was found. It was, in fact, the substance glyoxaline itself, which always occurred as long prisms having four faces in the prism-zone; the ends of the prisms were jagged and rough, and no siglls of optically reflecting-planes could be found on these ends. In such a case, X-ray methods of investigation are now available, and the problem in question is the determination of the dimensions of the unit-cell, not the complete X-ray inveatigation of the structure.

The spots of a Laue crystal-photograph may be regarded as due to the reflection of X-radiation by the planes of the crystal and the interpretation of these photographs involves the determination of the index of the plane by which any given spot is produced. This 'interpretation can be readily accomplished by two distinct though related methods. We may, in the first place, utilize the stereographic projection and determine the indices of the planes by means of intersecting zone- circles. This method, the earliest in point of time, was explained by W. L. Bragg in 1913. In recent years, however, it has become usual to employ for this purpoee the gnomonic projection which affords a solution in some respects impler and more direct than the stereographic method used by Bragg, and detailed illustrations of its application in this field have been given by R. W. G. Wyckoff, and E. Schiebold.

Gold and silver have been detected not only in the peculiar facies of the granite occurring on Bittleford Down, but also in the normal grey biotite-granite of numerous tor- and quarry-localities; several hand-specimens of granite showing minute flakes of visible gold have been collected.

The modes of occurrence of gold on Dartmoor, the environment of the granite from which specimens showing visible gold have been taken, and the gold-silver content of the granite generally have been investigated in some detail, with the results described below.

In this paper further evidence is put forward in support of a previous paper on the subject. Very soon after the reading of that paper, A. E. Mitchell read a paper before the Chemical Society on a closely related subject. Working quite independently, and employing other methods of investigation, Mitchell arrived at conclusions in close agreement with those detailed in the former paper. H.L.J. Bäckström has criticized Mitchell's paper, raising some objections 'against the experimental evidence produced aud the … way of interpreting it'. Where Bäckström objections touch on anything in Mitchell's paper which is incidentally akin to any of the work described in this or the earlier paper, they will be discussed herein.