April— -sept. 1859.] the Province of Auckland. 



121 



dered as of igneous origin, formed in the earliest periods of the 

 earth, in great depths, and cooled and crystalized slowly under 

 great pressure. 



The Metamorphic rocks are the crystalline strata, or schists, 

 called gneiss, mica-schist or mica-slate, chlorite- schist, hornblende- 

 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 

 subterranean heat, so as to assume a new texture. 



The two first classes of rocks are usually found in such a posi- 

 tion that they form the foundation on which the aqueous rocks 

 were afterwards superimposed. For instance, they compose the 

 central line of a range of mountains, on both sides of which sedi- 

 mentary rocks are deposited. Thus, in reference to their age, they 

 are considered as the oldest, and are therefore called also Primitive. 



There are exceptions to this rule in reference to the age of cer- 

 tain plutonic rocks of eruptive character. But I am now stating 

 only general principles, and therefore avoid all questions leading 

 to scientific discussions. 



The next in order are the aqueous rocks — the production of 

 watery action. They are also called sedimentary rocks, from the 

 fact that they are the hardened sediments accumulated at the bot- 

 tom of the sea or of fresh-water lakes. They are stratified, or 

 divided into distinct layers or strata : as, for example, clay-slate, 

 marl, sandstone, limestone, and are divided into three kinds, called 

 arenaceous or siliceous, argillaceous or clayey, and calcareous or 

 chalky — according to the respective predominance of Silica, Alu- 

 mina, or Lime. Hocks of this class cover a larger part of the 

 earth's surface than any others, and are of the greatest interest on 

 account of the organic remains which are found imbedded in the 

 different strata. 



There are two principal means of ascertaining the relative age 

 | of aqueous rocks—derived, the one from their position, the 

 j other from the fossil remains they contain. 



With reference to position — the bed which lies uppermost, is of 

 course the newest of all, and that which lies at the bottom, the 

 most ancient. 



Vol. xx. o. s. Vol. vi. n. s. 



