322 SECTIONAL TRANSACTIONS.— C. 



Ancient sediments are represented amongst the crystalline rocks of the Port Herald 

 Hills. The presence of sheared conglomerates and quartzites and of graphitic gneiss 

 and magnetite-quartzites, associated with foUated hornblendic and other rocks, 

 suggests a close relation to the crystaUine schists of the southern part of Mozambique ; 

 while in the association of thick beds of white crystaUine limestone with a wide range 

 of metamorphosed sediments the rocks resemble the Turoka series and related rocks 

 of Kenya and Tanganyika. Quartz-schists, quartz-felspar-schist and quartz- 

 magnetite granulite showing var3ring degrees of metamorphism, together with 

 crystaUine limestone, are recorded also from amongst the oldest known rocks of 

 northern Mozambique. 



In addition to biotitic gneisses, pegmatite and basic dykes, the intrusives include 

 large masses of nepheUne-syenite resembUng those from other parts of Nyasaland 

 but hitherto unrecorded from Portuguese East Africa. 



The Karroo has 3'ielded results of especial interest. On palaeontological and 

 stratigraphical grounds it has been possible to recognise the Ecca, Lower Beaufort 

 and Stormberg series. Beds containing Rhexoxylon africanum rest directly upon beds 

 assigned to the Lower Beaufort, and accordingly the upper divisions of the Beaufort 

 appear to be absent. The volcanic rocks include basalts with interbedded rhyolitic 

 rocks, comparable with those of the Lebombo mountains in the southern part of 

 Portuguese East Africa. 



The Lupata series rests upon a deeply-eroded surface of the Karroo and older 

 rocks. The lavas include an earUer rhyoUtic series and a later unconformable alkaline 

 series. The Upper Lupata Sandstones can be foUowed along the north bank of the 

 Zambezi from the Lupata Gorge to the hiUs north of Mutarara, where the rhyolitic 

 series reappears. The Lupata Sandstones, recognised also in the Lower Shire VaUey, 

 closely resemble the Dinosaur beds of North Nyasa, and in this and other respects 

 recent work tends to confirm the view that the Lupata series dates back to the 

 beginning of the Cretaceous. 



The Tertiary is represented by intrusions of Umburgite and oUvine-nephelinite 

 into the Mutarara sandstones. 



Dr. G. Slater. — The Princifles of Glacial Tectonics. 



The combined study of Arctic and Alpine glaciers on the one hand, and disturbed 

 Pleistocene Drift deposits on the other, proves that obstruction to movement of a 

 glacier results in the development of a definite tectonic structure. In Arctic glaciers, 

 englacial material is incorporated in the structure, which, however, is largely destroyed 

 during the formation of the marginal moraines, owing to the rapid rate of melting of 

 the ice. On the other hand, in disturbed Pleistocene Drift deposits, lenticles or 

 sheets of the local country rock, such as chalk or clay, have been frozen into the basal 

 layers of the ice as englacial material. This material, after transportation, was 

 subsequently moulded in tectonic forms which have been preserved as pseudomorpha 

 after the melting of the interstitial ice. Classical examples of such deposits occur in the 

 Norfolk clifEs of England, Moen and Lonstrup (Denmark), Rugen (Germany), and in the 

 Mud Buttes of Alberta (Canada). The tectonic forms may be divided into two 

 classes : — • 



I. The Roche-moutonnee Type. 



The roche-moutonnie is associated with the forward movement of the ice. It may 

 be carved out of the country-rock or may be moulded from the stranded englacial 

 material of an overloaded ice-sheet. In modern glaciers, and in disturbed drift depos- 

 its, the material overlying the roche-moutonnee ' kernel ' shows thrUst-planes on the 

 iceward-limb, tip-structure on the leeward-Umb, and the development of a graceful 

 asymmetrical or drumloid curve when seen in longitudinal section. Traced in the 

 direction of movement of the ice a series of roche-moutonnees may be developed 

 proving a switch-back mode of progression. In mountainous areas the ' kernel ' is often 

 the only part of the structure preserved. 



The succeeding stage is shown by the accretion of transported material on the 

 iceward-flank of the roche-moutonnee. This material is associated with pressure- 

 gradients marked by a festoon of fan-shaped thrust-planes. The climax is shown by 

 the development of a highly inclined plane which in cross-section forms a sigmoid- 

 curve. The final result is the formation of a ' horst ' of drumlin-form. 



