Ch. xxviii] cokes and ceaters. 461 



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

 as at a, fig. 620. But, secondly, the step-like appearance arises more 

 frequently from tlie mode in which hori- 

 zontal masses of igneous rock, such as h c, ^^' 

 intercalated between^ aqueous sti'ata, or 

 showers of volcanic dust and ashes, have, 

 subsequently to their origin, been exposed, 

 at different heights, by denudation. Such 

 an outline, it is true, is not peculiar to 



trap rocks; great beds of Umestone, and stcp-iike appearance of trap. 

 other hard kinds of stone, often presenting 



similar terraces and precipices ; but these are usually on a smaller scale, 

 or less numerous, than the volcanic ste2)S^ or form less aecided features in 

 the landscape, as being less distinct in structure and composition from the 

 associated rocks. 



Although the characters of trap rocks are greatly diversified, the be- 

 ginner 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 by horizontal planes of stratification 

 in the manner of sedimentary deposits. Sometimes they form chains of 

 hills often conical in shape. Not unfrequentl}^ they are seen as " dikes " 

 or wall-like masses, intersecting fossiliferous beds. The rock is occasion- 

 ally columnar, the columns sometimes 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 color, from the 

 oxidation of ferruginous matter, so abundant in the traps in which augite 

 or hornblende occurs; or, in the felspathic varieties of trap, it acquires a 

 white opake coating, from the bleaching of the mineral called felspar. 

 On examining 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 texture of the mass is 

 cellular or porous, or we perceive that it has once been full of pores and 

 cells, which have afterwards become filled with carbonate of lime, or 

 other infiltrated mineral. 



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

 tion. It seems that their component ingredients, silica, alumina, hme, 

 potash, iron, and the rest, are in proportions well fitted for the growth of 

 vegetation. As they do not effervesce with acids, a deficiency of calca- 

 reous matter might at first be suspected ; but although the carbonate of 

 lime is rare, except in the nodules of amygdaloids, yet it will be seen that 

 lime sometimes enters largely into the composition of augite and horn- 

 blende. (See Table, p. 475.) 



Cones and Craters.— In regions where the eruption of volcanic matter 

 has taken place in the open air, and where the surface has never since 

 been subjected to great aqueous denudation, cones and craters constitute 

 the most striking peculiarity of this class of fl)rmations. Many hundreds 

 of these cones are seen in central France, in the ancient provinces of 



