Canterbury and Westland. 341 



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 dis- 

 tinguished French chemist, the uppermost portion is occupied by the 

 acidic magma, which, besides 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, the 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 immediately 

 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 

 understand 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 suggestion 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. li. Mallet, in the " Transactions of the 

 Eoyal Society (Phil. Trans. 1873,") has proposed another theory, 

 namely, that the principal cause of vulcanicity is to be sought in the 

 compressing and crushing action taking place beneath the crust of the 

 earth, and by which such a great amount of heat is generated that a 

 fusion of rocks, often on a large scale, is easily produced. This theory 



