XV. — GEOLOGY AND MINERALOGY. 137 



ores, the latter of those which, by their clecoinposition and 

 disintegration, are capable of producing fertile soils. It is 

 also by studying the rocks that the architect becomes ac- 

 quainted with the properties of building materials, their resistance 

 to weathering, &c., and the manufacturer with the elements of 

 useful or new materials such as bricks, artificial stones, &c. 



The general collection, regarded as a standard collection, will 

 serve as a comparison between the rocks of New South Wales, 

 which are yet sparingly represented, and the typical rocks shown 

 from different parts of the world. In each class the Australian 

 rocks will be found intercalated after the types and arranged in 

 groups by themselves. 



Two large subdivisions have been adopted. The sedimentary 

 rocks which occur in strata, the most ancient of which are 

 frequently upturned and much contorted, as well as meta- 

 morphosed ; and the eruptive rocks which occur as bosses, dykes, 

 volcanic lavas, &c. 



In the sedimentary series are found the Micaschists, Slates, 

 Sandstones, Conglomerates, Breccias, Limestones, Gravels, »fec. 

 Fossils are often seen in them. 



The eruptive series deserve a close study for various reasons. 

 They have often been associated with great disturbances in the 

 strata of the sedimentary rocks, and some of them have played an 

 important part in the filling of mineral veins, and they contain 

 various crystallized minerals. They can be divided into three 

 broadly defined groups, which must be understood to merge into 

 one another. The Acidic rocks, which contain a large propor- 

 tion of silica or quartz, Ex. : Granite ; the Basic rocks which 

 contain the least quantity of silica, and consequently the largest 

 amount of bases or oxides, some of these being metallic oxides, 

 such as iron, which give to the generality of these rocks a dark 

 colour, Ex. : Basalt ; between these two extremes and connecting 

 them are the so-called Intermediate rocks, containing in general, 

 less silica than the Acid, and more silica than the Basic rocks, 

 Ex. : Syenite. 



