90 LYELL'S ELEMENTS OF GEOLOGY. 



General Appearance of Volcanic Rocks. 



unequal extent, so as to form a succession of terraces, or steps, 

 on the sides of hills. This configuration appears to be derived 

 from two causes, first, the abrupt original terminations of sheets 

 of melted matter, which have spread, whether on the land or 

 bottom of the sea, over a level surface. For we know, in the 

 case of lava flowing from a volcano, that a stream, when it has 

 ceased to flow, and grown solid, very commonly ends in a steep 

 slope, as at o, Fig. 85. But, secondly, the step-like appearance 

 arises more frequently from the mode 

 Fig. 85. m which horizontal masses of igne- 



ous rock, such as b c, intercalated 

 between aqueous strata, have, sub- 

 sequently to their origin, been ex- 

 posed, at different heights, by denu- 

 dation. Such an outline, it is true, 

 is not peculiar to trap rocks : great 



Step-Me appearance of trap. ^ other 



kinds of stone, often presenting similar terraces and precipices ; 

 but these are usually on a smaller scale or less numerous, than 

 the volcanic steps, or form less decided features in the landscape, 

 as being less distinct in structure and composition from the asso- 

 ciated rocks. 



Although the characters of trap rocks are greatly diversified, 

 the beginner will easily learn to distinguish them as a class from 

 the aqueous formations. Sometimes they present themselves, as 

 already stated, in tabular masses, which are not divided into 

 strata, sometimes in shapeless lumps and irregular cones, form- 

 ing small chains of hills. Often they are seen in dikes or wall- 

 like masses, intersecting fossiliferous beds. The rock is occa- 

 sionally found divided into columns, often decomposing into balls 

 of various sizes, from a few inches to several feet in diameter. 

 The decomposing surface very commonly assumes a coating of 

 a rusty iron colour, from the oxidation of ferruginous matter, so 

 abundant in the traps in which augite or hornblende occur ; or, 

 in the felspathic varieties of trap, it acquires a white opaque coat- 

 ing, from the bleaching of the mineral called felspar. On exam- 

 ining any of these volcanic rocks, where they have not suffered 

 disintegration, we rarely fail to detect a crystalline arrangement 

 in one or more of the component minerals. Sometimes the tex- 

 ture of the mass is cellular or porous, or has been porous, and 

 the cells have become filled with carbonate of lime, or other in- 

 filtrated mineral, which has thus taken the globular form of the 

 cells. 



Most of the volcanic rocks produce a fertile soil by their dis- 

 integration. It seems that their component ingredients, silica, 



