124 FORMER LAND CONNECTIONS. 



Furthermore, there appeared in this half of the world, but 

 principally in South Africa, a wonderful assemblage of reptilian 

 forms, the morphological study of which has so brilliantly confirmed' 

 the doctrine of Evolution. 



The Glaciation of Gondwanaland. 



Before describing this amazing glacial episode of the Car- 

 boniferous epoch, just a few words are needed to explain why ; 

 geologists have both formulated and accepted views demanding, 

 such a radical change of climate over so vast a territory. 



Glaciers in moving down to lower levels carry with them rock 

 debris either resting upon their surfaces or embedded in the ice 

 itself. The inclusions in the ice-foot, pressed down upon the rock, 

 floor over which the ice is moving, polishes and scratches the latter, 

 hence the peculiar striated surfaces characteristic of areas formerly 

 over-ridden by ice-sheets. Upon the melting of the latter the clay, 

 sand and boulders are deposited upon the floor, this material being 

 termed "moraine" or "till." 



It is unbedded, possesses a clayey matrix, contains striated and 

 facetted boulders which are sometimes many feet in diameter and 

 are of various types of rock such as the ice has moved across; 

 occasionally they may have been transported for hundreds of miles. 



About half a century ago formations were observed in India, 

 South Africa, and Australia that pointed to glaciation, and, though 

 the opinions then expressed for quite a while failed to receive due 

 recognition, subsequent work has proved to the full with a wealth 

 of detail the correctness of this original interpretation. It will be 

 of no small interest for you to realise that the morainic character 

 of this deposit was recognised by Sutherland and by Griesbach in 

 these early days and that one of the critical localities cited is near 

 the Umgeni Bridge only a few miles north of Durban. The floor 

 is a finely grooved surface of hard white sandstone, while the over- 

 lying morainic deposit, now consolidated by age and pressure, is 

 the peculiar green-blue rock with pebbles and boulders used foir 

 macadamising the streets of Durban. 



The identical formation was later on observed by Dunn in the 

 Cape and its true nature recognised by him ; from its occurrence, 

 crossing the Dwyka River near Prince Albert, it received the name 

 of the "Dwyka Conglomerate," a term familiar to geologists the 

 world over. Since then it has been traced over a great area, and 

 either covers or else underlies nearly two-thirds of the Union, and, 

 while variable in thickness, exceeds a thousand feet in depth over 

 wide stretches. 



Movement of the Ice in South Africa. 



Wherever the conglomerate has been stripped by erosion from 

 the floor the latter is found to be uneven but striated ; the direc- 

 tion of the groovings reveals in addition the course of the ice move- 

 ment, while the nature of the boulders gives a clue as to the source 

 of the ice itself. 



With these as our guide it is possible to attempt the reconstruc- 

 tion of the past, and it has been made out that the principal ice- 1 



