Thomas. — Volcanic Rocks of the Taupo District. 307 



rocks collected by hini belonged to the family of rbyolites. The 

 rhyolites are a group of lavas which present themselves in an 

 immense variety of structure : they are characterised, by the 

 large proportion of silica which they contain, generally so large 

 that the excess separates out in the form of free quartz. 



The rocks collected by Mr. Cussen show, however, that a 

 more basic group of lavas, with a lower proportion of silica, are 

 abundantly represented in the Taupo District. The more recent 

 lavas of the giant volcanoes Euapehu, Ngauruhoe, and probably 

 Tongariro, appear to consist of the basic rocks known as augite- 

 andesites. These are richer in metallic bases and poorer in 

 silica than the rhyolites. 



Up to the time of the eruption of Tarawera, in June, 1886, 

 basic rocks were not known to occur in the Taupo volcanic 

 zone.* I have shown elsewhere that the lava of the Tarawera 

 eruption was a form of augite-andesite, and that tbe same rock 

 occurs at Mount Edgecumbe, the volcanic cone lying 15 miles 

 north-east of Tarawera and in the direction of the Bay of Plenty. 

 It is interesting to find that the same rocks occur as the most 

 recent lavas on the great cones in the south of the zone, and 

 that at so many points along the main line of activity the suc- 

 cession of volcanic rocks has been the same — basic augite- 

 andesite succeeding the acid rhyolites. The country around 

 Lake Taupo is remarkable for the vast quantities of pumice 

 which form the superficial deposits ; but it is stated by Mr. 

 Cussen that on approaching Euapehu and Tongariro the pumice 

 becomes less abundant, and that the surface is formed by a layer 

 of dark brown loam, which is more fertile than is usually the 

 case with the soil of this region. That this is the case is shown 

 by the establishment of sheep-runs near Tongariro. The richer 

 character of the soil appears to be due to its origin from the 

 decomposition of andesitic ashes from the more recent eruptions 

 of the mountains. Speaking generally, the ashes of basic rocks 

 such as the augite-andesites, or basalts, yield by their decom- 

 position a richer, more fertile soil than the acid rocks. 



As is well known, the order of succession of the rocks at 

 any given vent, or in a given volcanic district, is such that the 

 more basic follow the acid lavas. The appearance of the basic 

 lavas at such different points in the Taupo zone seems to indicate 

 the opening of a new phase of its volcanic activity, this present 

 stage being characterised by basic lavas. There seems, therefore, 

 a probability that, if the country in the Taupo volcanic zone is 

 ever again covered with great showers of ashes like those which 

 recently fell at the eruption of Tarawera, those ashes will, as 



* Hochstetter, however, in " The Voyage of the Novara " (Geology : 

 vol. i., p. 1U4), states that he saw in the Museum of the School of Mines, 

 London, basalts which were said to be from White Island. 



