Haast. — On the Geological Structure of Banks Peninsula. 605 



those during wiiicli the caldera walls were built up. It is evident that a 

 great j)ortion of the lava-streams and agglomeratic beds which once formed 

 the crater of the volcanic system of Lyttelton Harbour, must have been 

 blown away, or at least removed during one of those violent outbursts of 

 subterranean forces necessary to clear the choked vent of the volcano, 

 similar to those by which in recent times the upper portions of active 

 volcanoes have repeatedly been destroyed under the eyes of the trembling 

 population in the neighbourliood. 



For an explanation we might go back to Durocher's views, that all 

 igneous rocks, even the most modern lavas, are derived from two distinct 

 magmas which co-exist below the solid crust of the globe, each of them 

 occupying a well-defined position. According to this distinguished French 

 chemist, the uppermost portion is occupied by the acidic magma, which, be- 

 sides being of lighter specific gravity, possesses a larger amount of silica 

 and less iron osyde than the other or basic magma. From the upper layer 

 the granites, porphyries, and trachytes, according to his views, are derived, 

 ^he zone of contact producing rocks of an intermediate character, such as 

 trachydolerites. If this theory is correct, we have to admit that not only 

 the dyke rocks were injected in rents formed during earthquakes, or imme- 

 diately before volcanic eruptions had taken place from the opened chimney of 

 the volcano, but that in each case the molten matter was furnished both from 

 the upper and lower stratum of incandescent matter below the hard crust of 

 the globe. There is, however, one great difficulty which crops up here, and 

 which I wish to point out, and that is the presence of dykes of basic rocks 

 and of others of an intermediate character. If all the radiating fissures 

 without exception had been filled up by acidic rocks, this would go far to 

 prove the existence of such an upper acidic incandescent magma ; in which 

 case we should be forced to the conclusion that the chimney of the volcano 

 reached lower down to the lower or basic layer. But it is difficult to under- 

 stand how all the radiating fissures over an area of 12 miles in diameter 

 could pass through the solid crust of the earth and through the fluid acidic 

 magma, and how the lower basic rocks could be injected into them from 

 below without disturbing the acidic magma, which certainly should have 

 been forced up before. This difficulty might, however, be met by the sug- 

 gestion that the radiating fissures in this instance did not reach so far down 

 as the fluid acidic magma, and that the material for the formation of the 

 dykes had been furnished from the crater itself, but it is scarcely conceivable 

 that for a distance of six miles and for an altitude of several thousand feet the 

 molten matter would have been forced in all directions from the central axis 

 of eruption along these fissures, often only a few feet wide. Mr, E. Mallet,* 



* Transactions of the Eoyal Society. Phil. Trans. 1873. 



