604 TRANSACTIONS OP SECTION C. 



4. Pleochroic Halos. By Professor J. Joly, F.R.S. 



An account of recent advances in the radioactive theory of the formation 

 of halos, and of evidence as to the chemical nature of the deposit occasioning 

 the halo. 



5. Outlines of the Geology of Northern Nigeria. 

 By Dr. J.'D. Falconer, M.A., F.G.S. 



The Protectorate of Northern Nigeria lies for the most part between Lake 

 Chad and the confluence of the rivers Niger and Benue, and comprises an area 

 of about 255,700 square miles. Crystalline rocks are exposed over about half of 

 this area, and among them two series have been recognised : (1) a series of 

 hard, banded, and much granitised gneisses of an Archoean type; (2) a series of 

 quartzites, phyllites, schists, and gneisses of sedimentary origin with associated 

 amphibolites, hornblende schists, and other more or less metamorphosed igneous 

 rocks. The two series, which were probably originally unconformable, have 

 been folded together along axes which are predominantly meridional in direction. 

 They have also been pierced by numerous igneous intrusions, which are readily 

 subdivided into an older and a younger group. The older group consists prin- 

 cipally of granites, wholly or partially foliated, which have been affected to a 

 varying extent by the forces which produced the metamorphism of the gneisses 

 and schists. The members of the younger group are non-foliated, and include 

 such types as tourmaline granite, riebeckite granite, augite syenite, augite diorite, 

 and numerous associated dyke rocks. 



Rocks of Cretaceous Age are found in the valleys of the Benue and the 

 Gongola and in the angle between the two rivers. They are invariably gently 

 folded and sometimes broken and faulted, and consist of a lower series of sand- 

 stones and grits, in part salt-bearing ; and an upper series of limestones and 

 shales, with numerous fossils of Turonian Age. The post-Cretaceous rocks, 

 which rest unconformably upon the Cretaceous limestone, and are probably all of 

 Eocene Age, occur over three detached areas : (1) in Sokoto province and the 

 Niger valley, (2) in Bauchi and Bornu, and (3) in Yola. The Sokoto series, 

 which contains marine intercalations yielding abundant Eocene fossils, is con- 

 tinuous southward with the sandstones, grits, and ironstones of the Niger valley. 

 The correlation of the sandstones, grits, and clays of Bauchi, Bornu, and Yola 

 with the Eocene rocks of Sokoto and the Niger valley is based partly upon 

 lithological similarities and partly upon the absence of evidence of any extensive 

 post-Eocene submergence of the Protectorate. 



Extensive fields of basaltic lava occur in Southern Bornu and on the borders 

 of Bauchi and Nassarawa; and numerous puys of trachyte, phonolite, olivine 

 basalt, and nepheline basalt are distributed throughout Southern Bauchi, Mini, 

 and Yola. The puys and lava fields alike are the product of Tertiary volcanic 

 activity. 



During the latter part of the Tertiary period there appear to have been 

 repeated minor oscillations of the crust, which culminated in the elevation of 

 the Bauchi plateau and the Nassarawa tableland, the depression of the Chad 

 area, and the establishment of the present river system. 



6. Notes on Natal Geology. By Dr. F. H. Hatch, F.G.S. 



Excepting the Cretaceous rocks, which only attain to a quite unimportant 

 development on the coast, there are three distinct formations which, separated 

 by great unconformities, take part in the geology of Natal. These are : — 



The Karroo System (Carboniferous to Jurassic). 



The Table Mountain Sandstone (Devonian). 



The Metamorphic Basement Rocks (Swaziland or Archaean). 



The horizontal beds of the Karroo System are of enormous thickness, and 

 extend over the greater part of the colony, from the Drackensberg to near the 

 east coast. Near the summit of the scarp of the Drackensberg they consist 

 of the Stormberg lavas and tuffs. These are underlain by the other divisions 



