GEOLOGY 39 



By ingenious methods of fractional extracts, elutriation, 

 etc., Mr. A. Brammall has been enabled to make a study of 

 the mineralogical changes that take place during the trans- 

 formation of shales to slates and phyllites (Min. Mag., 1920, 

 19, 211-24). The impure sericitic mica in shales, rich in mag- 

 nesia, lime, and iron oxides, is accompanied by an obscure 

 chloritic substance. The change to slates and phyllites is 

 marked by the establishment of a metastable ternary system of 

 white mica, chlorite, and quartz ; the ferromagnesian im- 

 purities temporarily accommodated in the sericite of the shale 

 supplying increment to the chloritic substance, which gains in 

 size and mineralogical definition, while the mica becomes clearly 

 muscovitic. Free silica is a by-product of this mineralogical 

 differentiation. 



Regional and Stratigraphical Geology. — A new book by Prof. 

 J, W. Gregory [The Geology and Rift Valleys of East Africa, 

 pp. 479, Seeley Service & Co., Ltd., 1921) deals with the 

 fascinating tectonic problems of the Great Rift Valley system 

 in particular, and also summarises the geology of the several 

 parts of East Africa from Egypt to Madagascar, paying especial 

 attention to British East Africa (Kenya Colony). The tectonic 

 structure of the Rift Valley system and its relation to contem- 

 porary earth movements is the main theme of the book. The 

 rifting began much earlier than is usually supposed. While 

 the movements took place mainly from Oligocene to Recent 

 times, there was a preparatory stage in the late Cretaceous. 

 The formation of the rifts was accompanied by great and long- 

 continued volcanic activity. 



Holtedahl's investigation of the rocks and fossils of the 

 Hecla Hook system in Bear Island, midway between Norway 

 and Spitsbergen, throws much light on Lower Palaeozoic palaeo- 

 geography in the American- Arctic region (Norsk Geol. Tidsskr., 

 191 9, 5, 121-48). On the basis of his own and the earlier 

 Swedish collections he has established that two fossiliferous 

 horizons are present in the Hecla Hook system, belonging 

 respectively to Middle and Lower Ordovician ; and that the 

 organisms are definitely American in character and affinities. 

 These horizons are correlated with the Black River and Canadian- 

 Ozarkian horizons of North America. These discoveries bring 

 the lower part of the Hecla Hook of Bear Island into correlation 

 with the Durness Limestone of the North of Scotland, and the 

 Porsanger and Varanger Series of Finmarken in Northern 

 Norway. Furthermore, the typical Hecla Hook of Spitsbergen 

 itself, concordant in strike, lithological characters, and strati- 

 graphical relations, with that of Bear Island, may now be 

 regarded as of Ordovician age. A note in Nature (April 29, 

 1922, p. 561) announces that A. Hoel has now definitely 



