CamMpPBELL.—On Crystalline Rocks. 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 felsitie 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 
_ Sverlong periods. Professor Judd has shown that in the extinct volcanic dis- 
triet of Schemnitz, in Hungary, 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 
_ “unterpart to those in the paleozoic in the British Isles. 
Most of the older eruptive rocks have been affected by metamorphic 
» Many intensely so; the vesicular kinds have had their cavities filled 
7 With minerals, often of extraneous origin, forming zeolites and geodes of 
ae gate, or by segregation, zeolites forming often constituent portions of 
ee bacalts, Chlorite, which always appears to have accompanied mineral 
a , is — present in considerable quantities in the older mem- 
