384 THE SEQUENCE OF LAVAS AT THE XORTH HEAD. [Dec. I914. 



In general, the thickness of each lava-flow is fairly constant. It 

 is certainly constant enough to warrant the conclusion that all the 

 different streams of lava flowed down the same slope, and pre- 

 sumably, therefore, they were emitted from the same orifice. In 

 every case a scoria -bed of some thickness covers the lava. The 

 material of the scoria is moderately coarse, as a rule, although that 

 which overlies the lowest phonolite is much finer than the rest. 

 The general coarseness necessarily indicates that the centre of 

 volcanic activity was not far away from the section displayed, and 

 this is true of all the different types of lava, except the trachy- 

 dolerite No. 21, which, as will be shown later, does not extend over 

 the length of the section. Only one distinct dyke is to be seen, 

 and it is formed of basalt. This dyke is situated at that part of 

 the cliff where the middle flow of phonolite rises to the summit, 

 and it extends to the top of the cliff at this point. 



The lava-flows appear to have suffered only to a slight extent 

 from denudation, and, with the exception of the middle phonolite, 

 the surface of each flow seems to have undergone very little 

 denudation before the succeeding flow of lava covered it. Beneath 

 the thick flow of basalt (21) is a boulder-bed, which, of course, 

 varies greatly in thickness. It generally consists in the main of 

 pebbles and boulders of phonolite, evidently derived from the 

 middle flow (14) : which flow, in the exposed section, is covered by 

 six other intermediate flows of basalt. This appears to indicate 

 that a considerable time elapsed between the outflow of basalts 

 20 & 21 ; though it must be allowed that on a volcanic slope 

 each lava does not necessarily cover the whole surface, and 

 at certain points where a particular lava-flow became heaped up it 

 may not have been concealed until many succeeding lavas had issued. 



All these volcanic rocks are younger than the Miocene, for they 

 rest upon sediments of this age near Dunedin. 



A detailed examination of the cliff showed that no rock can 

 actually be seen beneath the trachyte. The cliff at this point — in 

 the extreme south — is low, at a considerable distance from the sea- 

 beach, and is here covered with a growth of vegetation which to 

 some extent obscures the section : consequently, it is difficult to 

 form any certain conclusions about the relations of some of the 

 rocks one to the other. It appears to me, however, that an error 

 was made in my previous paper, in the statement that a flow of 

 basalt occurs beneath the trachyte. 1 



Above the trachyte is a bed of tuff, varying from 10 to 

 00 feet in thickness. This is followed by a flow of phonolite, 

 which is also of very variable thickness : at the southernmost 

 exposure it is about 50 feet thick, and is divided in the middle by 

 a bed of scoria 12 feet thick. Farther north, at the extreme 

 left of fig. 2 (p. 385), where the section becomes continuous, there 

 is but one flow of phonolite. and its thickness is only 10 feet. 

 Above this phonolite is a conspicuous bed of white tuff 8 feet thick, 

 but thinning out northwards. Upon this follow nine flows of 



1 Q. J. G. S. vol. lxii (1906) p. 418. 



