C. A. Cotton— Block Mountains in New Zealand. 281 



is arid; other consequent streams on the back slope of the block 

 are intermittent and feeble; and have not even begun to cut 

 gorges through the range. 



As the Pool Burn gorge (fig. 21) crosses the Eaggedy-Black- 

 stone block at a low sag in the crest line, there can be little 

 doubt that it is consequent on the deformation. Whether the 

 present transverse stream marks the point at which a conse- 

 quent lake in the Ida Yalley depression spilled over or whether 



Fig. 21. 



Fia. 21. The course of fhe Pool Bum. 



the excavation of the gorge kept pace with the deformation 

 cannot now be determined. 



The centripetal consequent streams of the Ida Burn and 

 Pool Burn systems are of insignificant size and in dry seasons 

 practically disappear. Both the Ida Burn and the Pool Burn 

 are fed by numerous wet-weather consequents which run down 

 the front of the Paggedy-Blackstone block and the back slope 

 of the Bough Bidge block; but the main volume of the Ida 

 Burn comes from the greywacke highland to the north and of 

 the Pool Burn from the schist plateau to the south. Their 

 courses are thoroughly graded on the weak material of the 

 overmass in the depression, broad valley plains have been 

 developed, and the streams are bordering on senility. How- 

 ever, the gradient of the valley floor of the Ida Burn is consid- 



Am. Jour. Sci.— Fourth Series, Vol. XLIY, No. 262. — October, 1917. 

 20 



