Revieivs — The rocks of S. Tome. 231 



events in the world's history, whether we consider the vast mass 

 of material concerned or the complexity of its relations. 

 Even now the area of injection covers at least 220,000 square 

 miles, and a considerable part of its original extent is doubtless 

 beneath the Indian Ocean ; vast numbers of the sills and sheets 

 are from 100 to 300 feet thick, while individual laccolithic swellings, 

 such as those of Ingeli and Insizwa, run up to 3,000 feet, and the 

 Andriesbergen sheet in the Queenstown district is 1,500 feet thick 

 over an area of 40 square miles. The intrusions show every possible 

 variety of form between flat sills and vertical dykes, as well as 

 laccoliths, bosses, and plugs, and often a sheet gradually turns down 

 and passes into a dyke-like body. In the Eastern Province of Cape 

 Colony and in Natal it is estimated that dolerites constitute about 

 20 per cent of the whole Karroo system, and in places the ratio 

 rises to 30 per cent. It is therefore obvious that such a phenomenon 

 is worthy of thorough investigation. 



A very striking feature is the uniformity of the rocks : apart 

 from the thick laccoliths, where there has been marked diSerentiation 

 into ultrabasic and intermediate types, they are all essentially 

 dolerites, eith er with or withoat olivine. In this paper only sufficient 

 petrographic details are given to elucidate the special points under 

 discussion. The author mainly deals with the tectonics of intrusion. 



It is shown that there was a close connexion with the eruption 

 of the Stormberg volcanics and probably with those of the Lebombo, 

 and all the phenomena are correlated with the diastrophic relations 

 of the great geo-synclinal depression that bounded Africa to the 

 south at the beginning of Jurassic time, and led immediately to the 

 folding and thrusting of the coast ranges bounding the Karroo area 

 to the south and w est, and presumably once also to the east ; the 

 Karroo is regarded as the " foredeep " of these chains. Within 

 this area the dolerites were intruded, the relative order of injection 

 of the sheets being from above downwards. It is suggested that 

 this process may be concisely described as " descensional lit-'par-lit 

 sloping ". 



The paper concludes with a comprehensive discussion of the 

 relation of the South African geological events of this period with 

 those of the other continents of the southern hemisphere. This is 

 too long to summarize here, but should be read by all petrologists, 

 since it gives a connected and correlated view of a series of events 

 of fundamental importance in terrestrial evolution. 



R. H. R. 



As EOCHAS DA Ilha de S. Tome. By A. Ferraz de Carvalho. 

 Publica96es do Museu Mineralogico e Geologico da Universidade 

 de Coimbra, No. 1. pp. 9-24, with 5 plates. 1921. 



T'HE Portuguese island of S. Tome, still only imperfectly 

 -^ explored from the geological point of view, lies in the Gulf 

 of Guinea, almost exactly on the equator and about 100 miles 



