This paper continues the study of the constituent minerals of the Dartmoor granite. The district to which it mainly refers comprises the for-area around Widecombe and a part of the aureole adjoining. No detailed reference will be made at this stage to aureole phenomena. The results to be recorded necessitate a brief description of the two main granite types and of their field relationships. Abundant varieties and modifications of both types occur, but for the Purposes of the present paper they are of subordinate importance.

Most of the for-masses and high-level exposures of granite in the area consist in the main of the type known locally as the ‘giant granite‘ — a very coarse-grained, strongly porphyritic roelc rich in biotite, and consistently garnetiferous; it has an index-figure 1 ranging from 7.5 to 12.

That dolomite (CaCO3 . MgCO3) on being heated behaves as though it were a mixture of the two carbonates by dissociating in two distinct stages, appears to be the widely accepted opinion. Thus it is staled (Mem. Geol. Survey, Special Reports on Min. Resources, London, 1920, vol. 16, ‘Refractory Minerals’, p. 102) ‘On calcination, dolomite undergoes a progressive dissociation which may be considered chemically as covering two distinct stages. The first effect of rising temperature is the expulsion of carbon dioxide from the magnesium carbonate contained in the dolomite and the formation of a mixture of magnesium oxide and calcium carbonate, accompanied by a considerable reduction in volume… . Complete calcination results in the expulsion of the remaining carbon dioxide, a still further reduction in volume, and the production of a dense crystalline aggregate of magnesium and calcium oxides.’

In this paper is described an apparently hitherto scientifically unnoticed mode of alteration of olivine-dolerite in the intrusive sill at Ible near Matlock.

The igneous rocks of Derbyshire, locally called ‘toadstones’, constitute a well-known basic series of both contemporaneous and intrusive rocks. They comprise lava-flows, sills, a few thin dikes, agglomerates, and tufts. All occur in beds of Lower Carhoniferous age, and are confined to the upper half of the known thickness of the Carboniferous Limestone, with, in the south of the county, the overlying Limestone Shales. They have been described, in recent years, by Sir A. Geikie (‘Ancient Volcanoes of Great Britain’, 1897, vol. 2); II. H. Arnold-Bemrose (Quart. Journ. Geol. Sot., 1894, vol. 50, pp. 603-644; 1907, vol. 63, p. 241, et seq.); and H. C. Sargent (ibid., 1918, vol. 73, pp. 11-25).

Recently A. Johnsen has given a graphical solution by means of the stereographic projection of the problem of determining the constants of a homogeneous shear of a crystal, knowing the initial and final indices of two faces or two edges.

The same problem may be solved more easily by means of the gnomonic projection.