352 Transactions. — Geology. 



slopes of Tongariro, to the north, on the other hand, pumice 

 is found to a depth ^Yhich may reach 2ft. or 3i't, but 

 often much less, or may be altogether wanting. The deposit 

 was probably thicker once, as some of the streams show small 

 terraces of pumice. This pumice is, however, only a superficial 

 coating : below we find ashes or lavas of more basic composi- 

 tion. On the lower slope of Tongariro, above Papakai, such 

 sections as the following may be observed in recent water- 

 courses : — 



Ft. 



1. Black soil ... ... ... ... i-l 



2. Subsoil, white and pumiceous ... ... -^-l^ 



3. White pumice .. . ... ... ... |-2 



4. Layer of loam, denoting an old soil and sub- 



soil, the upper Gin. darker ... ... 2 



5. Volcanic ash, fragments weathered brownish, 



dark-grey when broken open : towards 

 bottom in finer and coarser layers . . . 6-10 



6. Brown loam, sharply marked olf from the 



former : may represent an old land-sur- 

 face ... ... ... 1-4 



7. Breccia of fragments of lava, apparently 



augite-andesite. 



Examination of the deeper layers Nos. 5 and 7 shows 

 that they are composed of rocks related to the augite-andesites. 

 An analysis of the ash from layer No. 5, by Mr. Pond, shows 

 57'9 per cent, of silica. (See analysis No. 4.) The specific 

 gravity w^as 2-68, affording a similar indication. 



We may conclude, therefore, that the eruptions of Tonga- 

 riro for a considerable period have yielded lavas of interme- 

 diate (and basic) composition only. The pumice of acid com- 

 position which lies on the surface must have been derived 

 from some more distant source, and of course at a date subse- 

 quent to the great eruption of Tongariro which produced the 

 ash of layer No. 5. Seeing how widely the pumice has been 

 distributed in the district, we need not hesitate to make such a 

 supposition ; and I may quote as a parallel case the ashes from 

 the eruption of Tarawera which were deposited on the cone of 

 Mount Edgecumbe, at a distance of fifteen miles, to the thick- 

 ness of 14in. 



In concluding, I desire to express my indebtedness to Mr. 

 Percy Smith, Surveyor-General, for topographical information 

 relating to the Taupo district, and to Mrs. J. McCosh Clark 

 for the artistic sketches illustrating some of the geological 

 features of the district which is the subject of the present 

 paper. 



