324 



Transactions. 



The general surface of the floor of the middle of the basin consists of a 

 series of flat benches, sometimes half a mile in breadth, planed by stream 

 action from the less resistant sedimentaries, and covered with a thin veneer 

 of gravel either washed out of the upper beds, which are composed of 

 conglomerate, or derived from the encircling mountain ring and deposited 

 by the present streams when they ran at a higher level. This is bordered 

 on the eastern and western side, and to a certain extent on the northern 

 side, by limestone hills, which project through the floor to a height of several 

 hundred feet. The whole area is deeply dissected by streams which flow 

 in beds incised at times hundreds of feet below the general floor, and 

 bordered by narrow terraces cut in the Tertiary sediments and covered with 

 thin veneers of gravel (Plate XXI, fig. 2). When cutting through the lime- 

 stones the river-channels are narrow and gorge-like, and the sections thus 

 furnished enable a clear insight to be obtained into the structure of the 

 borders of the basin ; but the mantle of river-gravels renders the exact 

 interpretation of the central part a matter of extreme difficulty and a sub- 

 ject for speculation ; this especially applies to the farm land immediately to 

 the east of Castle Hill and to the country in the angle between the Thomas 

 River and Hog's Back Creek, one of the largest feeders of the Broken River 

 in its higher reaches. 



C. STRATIGRAPHY. 



1. Statements of Previous Workers. 



The most interesting problems suggested by a consideration of the 

 stratigraphy of the area are those connected with the question of con- 

 formity and the correlation of the beds. In order to have a basis of 

 discussion it will be best to state the respective opinions of McKay and 

 Hutton, and indicate the points on which they differed. McKay's classifica- 

 tion, no doubt influenced largely by the ideas of Hector, is as follows : — 



Table of Formations. 



The substantial discrepancies between this table of formations and that 

 of Hutton are — (1) the latter did not recognize the existence of a volcanic- 

 tuff bed below the lower limestone — that is, the Ototara limestone of 

 McKay ; (2) McKay correlated the chalk marls with the Amuri limestone, 



