Reviews — Geology in the Congo. 569 



in articles on special minerals, perhaps the most important being the 

 section dealing with zircon. Some varieties of this mineral show 

 considerable departures from the usual formula, Zr Og . Si Og ; 

 artificial crystals of the Composition 3 Zr Og . 2 Si Og have been prepared, 

 and it is suggested that some such isomorphous mixtures may occur 

 in nature. The variations in density and refractive^ indices of zircons, 

 and the effect of heat, light, and radium on stones of various colours 

 are discussed. Zircons can be divided roughly into two classes, red 

 and brown stones on the one hand and green stones on the other. 

 The green stones are usually less dense and have lower refractive 

 indices than the brown ones, and at present no satisfactory explanation 

 of these differences has been reached. As regards the colour, 

 Professor Doelter and others have shown that the brown stones are 

 decolorized when strongly heated, the colour returning under the 

 influence of radium emanations, but both heat and radium are without 

 appreciable effect on the green zircons. The inference drawn from 

 these facts is that the brown colour is due to a radio-active constituent 

 of the zircons, while the green stones owe their colour to some more 

 stable substance. 



VI. — Geology in the Congo. 

 Annales de la Societe geologique de Belgigue. Publications 



relatives au Congo Beige, 1912-13, pp. 75-125. 

 rPHE Geological Society of Belgium are to be congratula,ted on the 

 1 work which their members are doing in the Congo, and which 

 they are now publishing in an appendix to the annals of the Society. 

 The second part of this special publication contains details of a boring 

 passing through 80 metres of the Lualaba Series, observations on the 

 lower part of the Lubilache Beds, and a second series of M. Buttgen- 

 bach's contributions to the petrology of the district. 



M. Mercenier, working in the neighbourhood of Albertville on the 

 west of Lake Tanganyika, has traced a series of great faults running 

 approximately parallel to the Tanganyika depression. M. Cornet 

 has previously demonstrated the existence farther to the west of 

 another great depression some 200 kilometres -in length, which he has 

 called the Upemba " graben ". In a preliminary note by M. Delhaye 

 on the Katanga district we find evidence of yet another great series 

 of trough - faults causing the depression of the Lufira River. 

 M. Mathieu, in a paper on the hot springs of Lower Katanga, shows 

 that the distribution and characters of these springs bear a relation 

 to the areas of depression. He records the temperature and mineral 

 composition of these springs throughout the area, and shows that 

 these characters are closely related to their distribution. Thus the 

 hot springs of the Tanganyika depression have a temperature of 

 40-55° F., are highly charged with chlorides, sulphates, and soda, 

 and contain relatively little carbonate : those of the Upemba 

 depression, on the other hand, have a temperature of 70-100° F., 

 are not highly mineralized, and carbonates preponderate over 

 chlorides and sulphates. Springs of the first group appear to be 

 connected with eruptive rocks ; those of the second group occur 

 mainly at contacts of the quartzites with granitic rocks. 



