GEOLOGY, 281 



help in all questions bearing on the mode of origin of minerals, 

 and on the subsequent molecular changes which they have under- 

 gone. 



ROCKS consist of aggregations of one or more minerals. They 

 differ from each other in chemical composition, in texture and 

 structure, in origin and in age. Some have been produced by 

 the consolidation of materials from a molten condition, as, for 

 instance, the lavas of modern volcanoes. Such rocks are for the 

 most part strikingly crystalline ; they consist of crystals and crys- 

 talline particles of various minerals interlaced and bound together 

 into a compact mass. A second class of rocks has been formed 

 by deposition in water, as sand and mud are laid down at present 

 in lakes and the sea, as lime and iron are deposited by the waters 

 of many springs, or as fragments of shells, corals, and other 

 animals are washed together into a continuous layer on some 

 parts of the ocean bottom. Rocks of this second class show by 

 the fragmentary and usually more or less water-worn aspect of 

 their particles, or by their arrangement in successive layers of 

 deposit the aqueous conditions under which they were formed. A 

 third class unites many of the characters of the first and second 

 series. Some of its members, such as gneiss and schist, seem to 

 have been originally aqueous deposits sands, gravels, and clays 

 which being subsequently overlaid by other formations and 

 sinking deep into the mass of the earth's crust, have then been 

 compressed, heated, and crystallized, so as to assume new cha- 

 racters. Such rocks are known as Metamorphic. The whole of 

 the earth's crust, so far as it is accessible to human observation, 

 consists of rocks which may be referred to an Igneous, Aqueous, 

 or Metamorphic series. 



The study of the chemical and mineralogical character of rocks 

 forms the branch of Geology termed Petrography. Among the 

 apparatus for its prosecution the first place should probably be 

 given to carefully-selected and well-arranged collections of typical 

 rock specimens. These enable the inquirer to familiarise himself 



