Reports & Proceedings — Edinburgh Geological Society. 233 



basalts. Similarly, the later rocks of Iceland, and, to a lesser extent, 

 of Skye and the Small Isles, follow the same course. In space the 

 most remarkable variation is seen in the distribution of titanium, 

 the percentages of titanium oxide being high in the rocks of Greenland 

 and the Iceland Ridge, and falling away regularly on each side. 

 The Brito-Arctic Petrographic Province can be subdivided into five 

 regions, viz. the British, the Icelandic (including the Faroe Islands 

 and the Scoresby Sound district), the "West Greenland, the Jan Mayen, 

 and the Spitzbergen- Franz Joseph Land -Dickson Harbour, and 

 the differences subsisting between them are related to the processes 

 whereby the igneous activity was initiated. It is suggested that 

 a petrographic province consists of a number of adjacent regions of 

 igneous activity, in which similar rocks, or similar series of rocks, 

 have been produced, whence it follows that the processes by which 

 the magmas have been formed, differentiated, and intruded must be 

 similar, and the underlying materials on which these processes have 

 acted must also be similar. 



Dr. J. W. Evans: A General Proof of the Limitation of the 

 Symmetry Numbers of Crystals. On the assumption that crystals 

 are composed of cells identical in all respects, then, if n be the 

 degree of the symmetry of an axis and d an integer, the equation 



cos — = £ (l-d) must be satisfied. The only possible values of d are 



3, 2, 1, 0, the corresponding values of n being 2, 3, 4, 6. 



E. S. Pederov : The Numerical Relation between Zones and Faces 

 of a Polyhedron. The numerical relation shown by axes of symmetry 

 situated in planes of symmetry pointed out by G. Cesaro in 1915 

 is only a particular case of the more general one deduced by the 

 author in 1885. 



A. Ledoux, T. L. Walker, and A. C. Wheatley : The Crystallization 

 of Parahopeite. Crystals in the Royal Ontario Museum of Mineralogy 

 from the original locality, Broken Hill, North-Western Rhodesia, 

 are triclinic with the axial ratios a.: b : c — 0-7729 : 1 : 0"7124; 

 a = 93° 22', /3 = 91° 12', 7 = 91° 22'. Thirty-two forms are recorded. 

 The crystals have perfect cleavage parallel to the brachypinacoid, 

 and show lamellar twinning parallel to the macropinacoid. The 

 angle of optical extinction on the cleavage is 10° with reference to 

 the twin-lamellae. 



IV. — Edinburgh Geological Society (February 21, 1917). — "The 

 Submarine Contours around the Orkney Islands." By Dr. Flett, 

 L.L.D., F.R.S., President of the Society. Dr. Flett pointed out 

 that geologists were agreed that the main physical features of the 

 north-east of Scotland were developed long before the Glacial pei'iod, 

 and that the Orkneys were a northward continuation of the Caithness 

 plain which had suffered depression, and had been partly overflowed 

 by the sea. The principal sounds crossing the islands in a north-west 

 to south-east direction were the valleys of old rivers like those of 

 Caithness and Sutherland. At what date the land sank was not 

 definitely established, but Orkney was probably joined to Caithness 

 when the present assemblage of animals entered the islands. Along 



