SOS Transactions, -^^Oeology, 



the Lyttelton-Snmner road at riglit-angles, on the very summit of Evans 

 Pass, and which is repeatedly passed by the road winding in and out of the 

 different bays before reaching that pass. It can be followed to Taylor's 

 Mistake. Everywhere along the sea cliffs at and near the entrance of 

 Lyttelton Harbour, numerous dykes, mostly all in a vertical position, can 

 be seen pointing towards the centre of that harbour. A few, however, 

 stand in a slanting position, and others have a tortuous course. As one of 

 the remarkable changes which some of the dykes have undergone since their 

 formation, I may also mention one which is well exposed in the sea cliffs at 

 Ohinitahi, Governor's Bay ; here a dyke of domite, about nine feet broad, 

 crosses in a nearly vertical position the so-called trachyte sandstone de- 

 posited on the slopes of the quartziferous porphyry. After its solidifica- 

 tion, a new fissure, about three feet broad, has been formed parallel to its 

 direction, and running along its centre, which has been injected from below 

 by domitic matter, but slightly different from the former ; however, instead 

 of continuing to the top of the cliff, about twelve feet above the sea level, the 

 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 v/ill perhaps be useful if I offer a few remarks on 

 the causes which led to the formation of these remarkable dykes. I con- 

 sider 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, scori®, and lapilli were thrown out, together with lava- 

 streams which flowed in various directions. Before, or during these 



