266 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 192 8 



Permian, 2,400 to 3,400 feet thick, are in the main continental, 

 although marine and brackish water zones occur almost throughout 

 the whole of them. All the Paleozoic detritals appear to have come 

 from the east; to the west they are buried under the plateau lavas. 



The formations of the Franciscan trougli have not heretofore been 

 interpreted as of a geosyncline, but Branner's map of 1919, taken 

 together with the nature of the deposits, and the position east of a 

 high borderland during the Paleozoic, with the Amazon shield on the 

 west, show that we have here all of the structural elements of a 

 geosyncline. In a general way the center of this trough may be said 

 to lie east of Maranhao, and somewhat west of Sfio Paulo, with the 

 axis trending slightly southwest, and finally it extends along the coast 

 to Rio Grande do Sul. 



Since Wegener holds that South America all through tht Paleo- 

 zoic and most of the Mesozoic lay closely adjacent to Africa, the 

 Franciscan geosyncline must find its continuation northeastward 

 through western Nigeria. Now let us see what is known of the gen- 

 eral geology of western Africa, depending upon Lemoine, Chudeau, 

 and Krenkel.^^ As one proceeds from the Mediterranean southward 

 across West Africa into the Gulf of Guinea, the dated geology be- 

 comes more and more obscure, and this is mainly because one passes 

 from younger rocks and the depths of ancient Tethj's toward older 

 ones and finall}' up against the pre-Cambrian shield of central Africa. 

 This shield also extends widely through southern West Africa west 

 to Liberia and Sierra Leone. In a general way, we can say that the 

 Paleozoic seas of the north transgressed southward upon this very 

 ancient nucleus of Africa, with the southernmost shore extending 

 from about western Liberia, thence striking northeast to north of 

 Nigeria, and so on into the southern Sahara, where it is lost. It is 

 well known that there are much folded and metamorphosed schists 

 south of this old coast, and especially in the Gold Coast state; these 

 have sometimes been referred to the early and middle Paleozoic, but 

 Krenkel points out that they are more probably of Proterozoic age. 

 Accordingly we see that the structure and the dated sediments of the 

 Franciscan geosyncline of Brazil abut directly against the old 

 nucleus of Africa, and no continuation at all occurs of this South 

 American trough. On the other hand, we gladly grant that the 

 Tethyan overlaps upon Africa, having east and west strikes, and 

 with faunas of the Atlantic-Mediterranean realm, strikes into the 

 Amazon valley, but all paleontologists who have studied these Silu- 

 rian, Devonian, and Pennsylvanian faunas of Brazil have pointed out 



" The best general statement, with maps, is by Paul Lemoine : Afrique occidentale. 

 Handbuch d. regionalen Geologic, vol. 7, pt. 6A, 1913, pp. 1-88. A later work treating 

 of the tectonics is by R. Chudeau : Recherches sur la tectonique de r Afrique occidentale, 

 Bull. Soc. G^ol. de France (4), vol. 18, 1918, pp. 59-87. E. Krenkel, Geologie Afrikas, 

 vol. 1, in Geologie der Erde, 1925. The detail of the geology of West Africa is to appear 

 in the second volume ; all that we have here is a much generalized account on pp. 46-47. 



