Haast.—On the Geological Structure of Banks Peninsula. 505 
those during which the caldera walls were built up. It is evident that a 
great portion 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 
voleanoes have repeatedly been destroyed under the eyes of the trembling 
population in the neighbourhood. 
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 oxyde 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 thatin 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 voleano 
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 havo 
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. R. Mallet,» | 
* Transactions of the Royal Society. Phil. Trans. 1873. 
