Campbell. — On Crystalline Hocks. 455 



only from granite in texture. Trachytes are volcanic rocks, possessing pre- 

 cisely the same chemical constitution as felstone, and form their modern 

 representatives ; they occur largely in the central portions of this island. 

 Both belong to the acidic group, and form far more extensive deposits than 

 the basic, which are represented by melaphyres and basalts, and to these 

 belong the Auckland Isthmus volcanic rocks, while intermediate forms 

 termed trachydolerites (Scrope) predominate in some areas. This preponder- 

 ance of siliceous kinds has caused some geologists to consider that they 

 predominated in the older, and the basic in the more modern rocks. The 

 eruptions from the greater number of the active volcanoes of the present 

 day have apparently a basic character, but the recent investigations of the 

 nature of the bed of the ocean show that while Globigerina ooze covers the 

 ridges and plateaux down to 2,000 fathoms, lower deposits are covered 

 with a red clay, formed of decomposed felsitic minerals with particles of 

 highly vesicular felspar and pumice, and concretionary nodules of 

 manganese, a large proportion of which must be derived from submarine 

 eruptions ; thus while comparatively circumscribed deposits of augitic lava 

 are accumulated around the volcanoes, the more siliceous portions, com- 

 prising the ash and vesicular felsitic scoria, are accumulated separately on 

 the bottom of the ocean. Lyell, in his "Elements of Geology," mentions 

 that it can by no means be inferred that trachytes predominated at one 

 period of the earth's history and basalt at another, for we know that 

 trachyte lavas have been formed at many successive periods, and are still 

 emitted from many active craters ; but it seems to me that felspathic 

 lavas have generally preceded augitic when a volcanic action has extended 

 over long periods. Professor Judd has shown that in the extinct volcanic dis- 

 trict of Schemnitz, in Hvingary, lavas of an intermediate (acidic and basic) 

 character preceded outbursts of extremely acid, and then of extremely basic 

 character; the tertiary andesitic eruptions of Hungary forming an exact 

 counterpart to those in the palaeozoic in the British Isles. 



Most of the older eruptive rocks have been affected by metamorphic 

 action, many intensely so ; the vesicular kinds have had their cavities filled 

 with minerals, often of extraneous origin, forming zeolites and geodes of 

 agate, or by segregation, zeolites forming often constituent portions of 

 basalts. Chlorite, which always appears to have accompanied mineral 

 changes, is generally present in considerable quantities in the older mem- 

 bers of these rocks. There is also generally more lime, the potash and soda 

 having been more readily dissolved out than the lime. The rock termed 

 serpentine occurs with schists, and also as an intrusive rock, and appar- 

 ently is usually the result of decomposition of olivine rocks — dimagnesian 

 (ferrous, etc.) silicates — similar to the New Zealand dunite, or of mate- 

 rials derived from their disintegration. 



