Ch. XXVIII.] CONES AND CRATERS. 593 



ing from a volcano, that a stream, when it has ceased to flow, and 

 grown solid, very commonly ends in a steep slope, as at a, fig. 673. 

 But, secondly, the step-like appearance 

 arises more frequently from the mode in Fig. 67^ 



which horizontal masses of igneous rock, 

 such as b c, intercalated between aqueous 

 strata, or showers of volcanic dust and 

 ashes, have, subsequently to their origin, 

 been exposed, at different heights, by de- 

 nudation. ' Such an outline, it is true, is 



not peculiar to trap rocks ; great beds of Step-like appearance of trap. 



limestone, and 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 steps, or form 

 less decided features in the landscape, as being less distinct in struc- 

 ture and composition from the associated 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 by horizontal planes 

 of stratification in the manner of sedimentary deposits. Sometimes 

 they form chains of hills often conical in shape. Not unfrequently 

 they are seen as " dikes " or wall-like masses, intersecting fossiliferous 

 beds. The rock is occasionally columnar, the columns sometimes de- 

 composing 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 occur ; or, in 

 the felspathic varieties of trap, it acquires a white opaque 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, lime, 

 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 

 calcareous matter might at first be suspected ; but although the carbon- 

 ate 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 hornblende. (See Table, p. 608.) 



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

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

 since been subjected to great aqueous denudation, cones and craters 

 38 



