TRANSACTIONS OF SECTION C. 655 



octahedron angle of alum is not, therefore, as appeared from the ohser rations of 

 PfalF and Brauns, subject to any variation. 



The angle at which a given vicinal plane is inclined to the octahedron face is 

 independent of the area of the plane, and of the temperature of the solution and 

 of the barometric pressure : it appears to be conditioned by the concentration of 

 the solution at the surface of the plane. 



In confirmation of this view it is found that the upper and lower portions of an 

 octahedron face which stands vertical are often replaced by two different triangular 

 pyramids ; also that the three faces of one such pyramid are at a given moment 

 not necessarily equally inclined to the octahedron face which it replaces. 



When, as is often the case, one of the three vicinal planes is large and the 

 other two are too small to give a visible reflexion, the face appears to be a single 

 reflecting plane. It is this which has been mistaken for the octahedron face in 

 previous observations. 



Similar phenomena of growth are exhibited by crystals of other substances 

 belonging to different systems. The conditions of equilibrium between the crystal 

 and the solution are such that vicinal planes appear in place of simple forms ; 

 these vary with the concentration of the solution, and give rise to variations in the 

 measured angles which are only apparently anomalous. Their true position can be 

 determined on a crystal of cubic symmetry (such as alum) whose theoretical angles 

 are known. 



A further study of the faces developed during the growth of crystals will, it is 

 hoped, lead to a better understanding of the reasons why a simple face like the 

 octahedron should not be a surface of equilibrium, and of the relation between 

 the vicinal planes and the structure of the crystal. 



4. A Compaj'isoii of the Pebbles in the Trias of Budleigh Salterton and of 

 Cannock Chase. By Professor T. G. Boxney, D.Sc, LL.D., F.R.S. 



The pebbles in the two deposits correspond in certain respects. In both vein 

 quartz and various quartzites are abundant, with certain dark green rocks, which 

 undoubtedly in some, probably in most, cases owe their colour to minute 

 tourmaline. The compact quartzites so common in Staffordshire are found in the 

 Devonshire deposit, but less abundantly, while other quartzites more rare in the 

 former are commoner in the latter. A quartz-felspar grit like the Torridon 

 Sandstone occurs in both. The following are points of difference. The shape of 

 the Staffordshire pebbles is nearer to a prolate spheroid, that of the Devonshire to 

 an oblate one : igneous rocks are much rarer in the latter, and the compact 

 tourmaline rocks in the former. The author thinks that the pebbles at Budlei"-h 

 Salterton must have travelled from a more or less south-westerly quarter, and 

 that the frequent correspondence of materials indicates that somewhat similar 

 detrital rocks fringed the archoean rocks of the ancient western land in localities 

 far apart. 



5. On a Soda-Felspar Rock at Dinas Head, North Coast of Cornwall. 



By Howard Fox. 



Dinas Head adjoins Trevose Head, four miles west of Padstow. 



The base and foreshore of tlie headland appear to be entirely composed of 

 greenstone containing much calcite, probably an altered dolerite. 



Between the greenstone and the slate, as well as interbedded with the slate, 

 occurs a rock which covers about an acre. It assumes various characters, all of 

 which contain nearly 10 per cent, of soda and from 044 to G6-6 of silica. The 

 compact varieties are crypto-crystalline, and might easily be mistaken for cherts. 

 The concretionary and spherulitic varieties show grains and blades of a felspar 

 which is doubtless albite. It varies in colour from creamy grey to light brown and 

 dark bluish-grey ; it weathers white, and is often studded with cavities filled with 



