Speight. — The Geology of Banks Peninsula. 389 



process of time, as the dissection becomes more advanced, it will result in 

 the division of the ridge into a series of isolated blocks and thus aid in 

 the destruction of the cone. 



The more energetic attack on the heart of the volcano owing to the 

 bunching-together of the heads of the streams accounts for the form of 

 the remnants of dissected cones, which consist usually of a central resistant 

 plug forming the neck, and then some distance away isolated remnants 

 of the lava-flows in their proper positions. 



While the streams flowing on the outside surface of the cone are usually 

 consequent in character, with a tendency to subsequent tributaries, those 

 on the inside of the crater will have the character of obsequent streams 

 with steep grade, obstructed by falls and rapids when they flow over ledges 

 of harder rock. In a normal crater the erosion of obsequent streams will 

 not be important while volcanic action is going on ; but if the crater is 

 enlarged by explosion or by breaching this erosion may achieve important 

 results, and specially so if there is any concentration of the drainage of 

 the internal slopes in the early stages. This will naturally arise in the case 

 of a cone not subjected to explosion if it has been breached by a lava-flow 

 from the summit. Thus a well-organized direction of drainage will be 

 promoted from the beginning, and it is very probable that a number of 

 calderas may owe their initial development to this circumstance. 



The direction of drainage from the enlarged crater will generally follow 

 the line of one of the consequent streams of the outer slope if it be not 

 breached, since a low place in the wall of the crater-ring will usually head 

 a valley. This is well exemplified at the present time by Ruapehu, for the 

 low part of the crater-ring faces the eastern side, and the drainage from 

 the basin on the top of the mountain will naturally concentrate towards 

 that point. In this case the depression in tl\e height of the wall may 

 be due to the accidents of explosion, or to the attack of the wall from 

 the outer slope, or to both causes. When this direction is once firmly 

 established erosion will tend to enlarge the interior of the crater by 

 sapping back the walls, the form of the crater being to a large extent 

 preserved, the steep scarp slopes of the interior being maintained in just the 

 same way as the scarps of tilted sedimentary strata are preserved. Thus 

 the form of the crater will be kept till it reaches the dimensions of a normal 

 caldera, and the barranco will be the valley due to the erosion of the con- 

 centrated drainage. Should any modification of the structure occur from 

 any cause, or should denudation expose material of different character from 

 that of a normal cone or with different stratigraphical arrangement, then a 

 departure from this form will ensue, an instance being seen at the head 

 of Lyttelton Harbour, where the sedimentary strata and rhyolite beds 

 of different arrangement from the remainder of the volcano have caused 

 modifications in the form of the resulting cavity. Should the crater-wall 

 be broken in more places than one by the explosion or by normal volcanic 

 action, or should several streams cut back their heads till they have invaded 

 the crater, then similar features will be produced along other lines and more 

 than one barranco may drain the caldera. 



If we take the conditions obtaining- now in the case of Lyttelton, we 

 observe that all along the northern side of the harbour the valleys have 

 the features of obsequent streams, and as they cut back their heads and 

 diminish their average grades they will retain this character, but with 

 diminishing characterization, as the grade becomes less pronounced. The 

 spurs in this portion of the harbour are usually terminated in steep cliffs 



