THE GEOLOGY OF AUCKLAND. 
11 
Bay of Islands, to the Missionaries, and to almost innumerable 
friends in Auckland. 
PRELIMINARY REMARKS. 
I cannot suppose that all my audience are acquainted with 
the first principles of Geology. I shall therefore be under the 
necessity, in order to make my report intelligible, of prefacing 
a few remarks upon the chief divisions of the geological for¬ 
mations. 
The various rocks, soils, and minerals which occur upon the 
surface of the earth, or at various depths beneath it—in one 
word, the materials of the “ earth's crust 1 ' —are classified, in the 
first place, with reference to their different origin, or, in other 
words, with inference to the different circumstances and causes 
by which they have been produced. They are divided into four 
great classes—— Plutonic, Metamorphic, jLgueous, and Volcanic 
rocks. Another mode of classification is with reference to their 
a a e _that is, to the comparative periods of their formation. 
Those divisions will be easily understood. 
The Plutonic rocks comprehend all the granites, syenites, 
porphyries, diorites —rocks which agree inbeinghiglily crystalline, 
unstratified, and destitute of organic remains—which are con¬ 
sidered as of igneous origin, formed in the earliest periods of the 
earth, in great depths, and cooled and crystalised slowly under 
great pressure. 
The Metamorphic rocks are the crystalline strata, or schists, 
called gneiss, mica-schist, or mica-slate, chlorite-schist, horn- 
llende-schist— also destitute of organic remains. According to 
the most probable theory, these strata were originally deposited 
from water in the usual form of sediment, but were subsequently 
altered by 7 subterranean heat, so as to assume a new textuie. 
The two first classes of rocks are usually found in such a 
position that they form the foundation on which the aqueous 
rocks were afterwards superimposed. Bor instance, they com¬ 
pose the central line or a range of mountains, on both sides of 
which sedimentary rocks are deposited. Thus, in reference to 
their age, they are considered as the oldest, and are theiefoie 
called also Primitive. 
There are exceptions to this rule in reference to the age of 
