96 abstracts: petrology 



Luster, splendid, almost submetallic and bronze-like. Cleavage, 001, 

 perfect; 010, good. Hardness between 2.5 and 3. Pleochroism, 7 = 

 green brown, /S = olive green, a = olive green. Absorption, fairly 

 strong, 7 >^>a. Refractive indices, 7 = 1.704 ± 0.003, /3 = 1.685 ± 

 0.003, a = 1.610 ± 0.003. Birefringence strong. 2^Na = 42 to 69°; 

 2 Ei^ = 34 to 60°. Axial dispersion strong, 2Ev >2£'^. Optical orienta- 

 tion, 6 = 7, a : /3 = 0° or a small angle not over 4°. F. E. W. 



PETROLOGY, — The composition of rockallite. Henry S. Washington. 

 Quart. J. Geol. Soc, 70: 294-302. 1914. 

 The paper is a chemical study of a unique aegirite granite from the 

 islet of Rockall, north of Ireland. Only three small specimens are 

 known, and that examined was generously given through Prof. J. W. Judd 

 by the Governors of the Imperial College of Science. A very complete 

 chemical analysis was made, which confirms in the general features one 

 made some years ago in England. It shows, in addition, the presence 

 of large amounts of zirconia and ceria — the amount of the latter being 

 next to the highest yet known for igneous rocks. By comparison with 

 the minerals present in the rock, it is shown that these two oxides be- 

 long to the pyroxenes, and the probability is pointed out that the pres- 

 ence of these two oxides is characteristic of acmite as contrasted with 

 their absence in the closely related aegirite. Further study of this 

 point will be undertaken when material from Norway, to be furnished 

 through the kindness of Professor Brogger, is received. H. S. W. 



PETROLOGY.- — The occurrence of molybdenum in rocks, with special 

 reference to those of Hawaii. John B. Ferguson. Am. Jour. 

 Sci., (4), 37: 399-402. 1914. 

 This article deals with the unexpected discovery of traces of molyb- 

 denum in two basaltic lavas from Hawaii and the question of the dis- 

 tribution of this element in igneous rocks. Its presence in the two ba- 

 salts reopens the question of its occurrence, since it was thought to be 

 confined entirely to the more siliceous rocks. Tests were accordingly 

 made on a trachyte obsichan from Hawaii, on some sodic, and especially 

 on some nephehte-bearing igneous rocks from other localities. From 

 these it would appear that the presence of molybdenum is not correlated 

 with high soda or potash content. Except for its well-known tendency 

 to occur in the more siliceous rocks, it therefore seems to be influenced 

 by regional rather than by general chemical characters. J. B. F. 



