112 PHYSICAL GEOLOGY. 



17. At a more mature period lie may disarrange the whole 

 collection, and endeavour to restore each specimen to its 

 place. 



CHAPTER VI. 



PHYSICAL GEOLOGY. 



Authors : Hutton, Playfair, M'Culloch, Bakewell, Lyell, Murchison, 



Delabeche, &c., &c. 

 Museums : Geological Society, Museum of Economic Geology. 



HAYING- furnished elementary information, respecting the 

 substances composing rocks, we now proceed to give an 

 outline of the nature and classification of the rocks them- 

 selves. The most comprehensive arrangement which the 

 student can form, is by reducing them to that fourfold 

 division, under which they may be classed as follows : 



I. After penetrating through the vegetable mould, or 

 alluvium, or drift, or other superficial deposit, the under- 

 lying rock, at certain points, is found to be a substance 

 of crystalline, or glassy texture, consisting of one or other 

 of the plutonic or unstratified rocks, comprising granite, with 

 its associate deposits of syenite, greenstone, hypersthene 

 rock, diallage rock, and serpentine ; or one of the trap rocks, 

 including basalt, porphyry, clinkstone, claystone, compact 

 feldspar, pitehstone, &c., &c., which are all characterised by 

 two distinctive features the absence of stratification and 

 of organic remains. 



II. In other localities the crust of the earth is formed of 

 volcanic rocks, comprising the lavas, trachytes, basalts, grey- 

 stones, obsidian, pumice, tufa, &c., the ejection of modern 

 volcanoes. 



III. In other places we meet, in the same relative posi- 

 tion, with the metamorphic or altered rocks, or primary 

 strata, as they are variously styled, which are conceived 



