OF VOLCANIC ROCKS. 61 



festation of subterranean energy, as some of the monuments of the grand volcanic 

 action of the past indicate the former existence, at certain places, of a stage intermedi- 

 ate between both. But in those numerous instances where the nature of either of 

 them can be distinctly recognized, some conspicuous differences may be noted between 

 them. Volcanoes are provided with a channel connecting the seat of volcanic action 

 with the surface. The matter which they eject consists either of stratified layers of 

 ashes and scoria, or of currents of lava in the shape of flat sheets superimposed over 

 one another, or of alternating layers of both kinds of material, the latter structure 

 being most frequent. A low dip, verging on horizontality, of all planes dividing sub- 

 stances of dissimilar nature, or marking successive deposition, and homogeneity of 

 material parallel to those planes, together with a structure of the mountain masses 

 distinguished by the radiation of all mineral matter from one common center, may be 

 considered as those features by which even extinct volcanoes, or fragments of them, 

 may unmistakably be recognized. The center may shift, or have shifted, within nar- 

 row limits, or a series of centers may follow each other in close succession, but this 

 will obliterate the true character only in a slight degree. Similar rocks, when they 

 came to the surface by massive eruptions, do not present these distinguishing features. 

 They usually compose ranges of small width in proportion to their length, and in the 

 place of one or more distinct centers an elongated axis may be detected, from which 

 the structural and morphological features originate. As regards the interior structure, 

 there may be observed a certain massive character of the rocks, which is partly pro- 

 duced by the prevalence of their compact varieties, and partly by the circumstance 

 that homogeneity of lithological character may be traced to a great distance in a verti- 

 cal direction. In sections, masses are frequently found thousands of feet in height, 

 which do not vary perceptibly in character, and show no horizontal structure. If the 

 slopes of the ranges are examined, the rocks will be found preserving a homogeneous 

 character chiefly in a direction parallel, to the axis, while it is less persistent at right 

 angles to the latter. The planes dividing dissimilar rocks are inclined at all angles, 

 and have very frequently a steep or nearly vertical position. -Breccias even, which 

 sometimes occur in very large masses, are bounded in this way towards the adjoining 

 compact varieties ; they are of irregular shape, and do not often occur in stratified 

 layers or in elongated currents, as is the case when they are produced by volcanic 

 action. The ranges made up by massive eruptions show no signs of craters. Yet 

 they are frequently the foundation of volcanoes. Oftener still do volcanoes occur on 

 the lower portions of their slopes, or they may form a series parallel to the axis of 

 the main range, and even greatly exceed it in elevation. 



Notwithstanding these points of difference, there are not only stages of transi- 

 tion between the distinct geological features resulting from either mode of action, but 

 a similarity in character may be produced under certain circumstances, which makes it 

 difficult to decide what was the mode of origin of an accumulation of volcanic rocks. 

 In the first place, the matter extruded through fissures may have been so liquid as to 

 expand at once in thin sheets. This is very frequently the case with basalt, in the 

 great accumulations of which, whether they be due to massive eruptions or to volcanic 

 activity, the prominent differences wrought by both modes of action in the external 

 M (99) 



