336 Geology of 



of the cliff, about twelve feet above the sea-level, the new dyke is seen 

 to turn from its vertical, to a nearly horizontal position, and to thin out 

 considerably at the same time, disappearing altogether when it touches 

 the side wall of the bed rock. The older dyke, above this change of 

 direction, is considerably shattered and broken. 



Before proceeding, it will perhaps be useful if I offer a few remarks 

 on the causes which led to the formation of these remarkable dykes. 

 I consider this the more important, as nowhere, as far as I am 

 aware, do they exist in such great numbers, nor do they possess 

 such a large longitudinal extent, as in the volcanic system under 

 consideration. It appears to me that the immediate cause of the 

 formation of a radiating system of dykes may be traced to the choked- 

 up vent or chimney of a volcano, the mouth of which, after an eruption 

 of considerable dimensions, is thoroughly filled up, either by its sides 

 falling in, by the cooling of ascending lava-streams, or by both causes 

 combined. When, from abyssological origination, masses of steam and 

 gases have collected below this vent, and new matter is ready to be 

 erupted, an enormous effort of nature will be necessary to clear out the 

 old, or form a new chimney, which cannot be accomplished without a 

 series of violent earthquakes, succeeded by an enormous explosion, by 

 which the mouth of the volcano is cleared out or newly formed, and of 

 the magnitude of which we can scarcely form a conception. A similar 

 effect, on a gigantic scale, must have been produced repeatedly by the 

 compressed masses of gases and steam during the formation of the 

 Lyttelton caldera wall, when the upper portion of the closed-up volcano 

 was not only removed, but vast quantities of ashes, scoriae, and lapilli 

 were thrown out, together with lava-streams which flowed in various direc- 

 tions. Before, or during these eruptions, molten matter in a high state 

 of fusion, generally rushed up in the fissures which had been formed at 

 the time, radiating from the focus like the spokes of a wheel. An exami- 

 nation of these dyke rocks will show at a glance that most of them are 

 quite different in composition and character from those of which the 

 lava-streams have been formed. The latter, as already explained, with 

 one notable exception, alluded to on page 332, all consist of true basic 

 rocks — basalts often assuming a doleritic texture, the dyke rocks being 

 generally acidic, having either the composition of a trachyte or domite. 

 "We are able to judge of the more or less high state of fusion in which 

 the molten matter ascended the open fissures from the effect produced 

 on the walls on both sides. The trachytic matter forming the dykes, 

 which are principally developed on the eastern side of the caldera wall, 



