524 Reviews — The Geology of Banks Peninsula. 



by low-lying marshy plains, and on the south-west by a large 

 shallow lake, Lake Ellesmere, which is separated from the sea by a 

 long narrow shingle spit. The volcanic rocks were to a great extent 

 poured out from two vents which, are now represented by the two 

 large calderas of Lyttelton Harbour in the western and Akaroa 

 Harbour in the eastern portion of the peninsula. , These calderas 

 have been converted into fjord-like inlets by erosion, and open on to 

 the north-west and south-east coasts respectively. Part of the 

 drainage of the region flows into the harbours, but most of it is 

 roughly radial, and the consequent streams flowing outwards from 

 the lips of the calderas enter the sea in a series of openings of the 

 ria type which indicate recent submergence. 



The geological foundations of the peninsula are a series of slates 

 and greywackes which probably belong to the Trias-Jura Maitai 

 system. On these rest the volcanics, which belong to four distinct 

 periods of activity. The rocks belonging to the first phase are 

 rhyolitic, and rest on the upturned edges of the older rocks; they 

 are nearly all lavas, with only one irregularly distributed fragmentary 

 deposit at the base. The vent was situated near Lyttelton, and, by 

 analogy with similar rocks elsewhere,. the eruptions probably took 

 place in Cretaceous times. The second phase was marked by the 

 building up of the two great volcanoes whose sites are occupied by 

 the calderas of Lyttelton and Akaroa Harbours. Both these 

 mountains were made up of basaltic material and were composite 

 cones of alternating lavas and fragraental deposits ; they were both 

 probably at least 10,000 feet in height. The lavas vary from fine- 

 grained basalts to rocks largely made up of felspar phenocrysts. 

 This volcanic phase was followed by a dyke phase. These dykes 

 are mostly trachyte, but some are andesite and basalt; they have 

 a roughly radial arrangement which allows the position of the vents 

 to be determined with tolerable certainty. The strike is not, 

 however, constantly radial, and mutual intersections of the dykes 

 are numerous. In the Akaroa area there is a large mass of coarse- 

 grained hornblende syenite ; this may be associated with the dykes, 

 but it is cut by some of them and is probably part of the original 

 land mass. 



Before the next volcanic phase the calderas had attained a form 

 not far removed from that wliich they have at present; during this 

 phase basaltic lavas were poured out from a vent in the neighbour- 

 hood of Mt. Herbert, about half-way between Lyttelton and 

 Akaroa, the date of the eruptions being probably Pliocene. 



The last volcanic phase was that which produced the basaltic 

 flows and ashes of Quail Island in Lyttelton Harbour. The island 

 is in the middle of the caldera and probably marks a final outburst of 

 this vent. 



Some time after this last event the land was depressed to an extent 

 of at least 700 feet, as may be shown by the occurrence of peat beds 

 at this depth in boreholes in the plains to the west, and this move- 

 ment has only lately given place to one of slight emergence. 



The author holds that the calderas are chiefly the product of 

 subaerial erosion, partly river and partly wave action. There must 



