CHAP. VIII.] TBIASSIC PERIOD. 



seem to point to the action of strong currents, such as would 

 only occur in an open sea or in the delta of a large river. 

 The average Staffordshire pebbles would require a current 

 of about three miles an hour to sweep them along. Larger 

 pebbles, up to six or eight inches in diameter, are by no 

 means infrequent.'* 



If we adopt the lake theory, we must suppose two long, 

 narrow, and shallow sheets of water, each receiving the 

 current of a rapid river at its northern end. To account 

 for the character and limited extent of the Lower Bunter 

 Sandstone, we must conclude that the rivers were at first 

 only of sufficient volume and velocity to carry sand into the 

 lakes, and that their sandy deltas gradually filled up the 

 spaces which were then covered by water. The wider ex- 

 tension of the Pebble Beds would indicate a rise in the level 

 of the lake-waters due to an increased winter rainfall ; an 

 increase which might be caused by continued elevation of 

 the country. We must assume that the rivers then became 

 equal to transporting pebbles over the same areas, and 

 that a pebbly delta was superimposed upon the sandy one, 

 gradually extending beyond the tracts covered by the lower 

 sandstones, and round the southern termination of the 

 Pennine range. The upper sandstones, if deposited in water, 

 would indicate a decrease in the velocity of the streams, 

 and a return to the former conditions, except that no such 

 deposit was formed in the eastern tract. 



This view of the manner in which the Bunter series was 

 laid down is not altogether satisfactory. Professor Bonney 

 says l he has always felt it difficult to explain the great 

 length of these Triassic tracts on the " filled-lake " theory, 

 and to account for the entire absence of mud or shale. He 

 thinks that the idea of subaerial river deltas, such as are 

 found in some parts of India, Persia, and Abyssinia, offers 

 a simpler explanation of some of the facts. On this theory, 



1 Address to Geol. Sect, of Brit. Assoc., 1886, p. 18. 

 K 



