OF UNSTKATIFIED ROCKS AND VEINS. 133 



in mountainous and hilly regions ; a remark very 

 conspicuously true in some parts of the enormous 

 ridges of South America. But granite is also found, 

 even on the level of the sea, in the same districts, as 

 are the different members of the trap family. The 

 theory which will hereafter be given respecting the 

 origin of these rocks, will be found to agree with this 

 mode of disposition. 



When the superficial disposition of the strata shall 

 hereafter be examined, it will be seen, that not only a 

 series of them, but a single bed, often occupies a very 

 considerable and continuous extent of surface ; and 

 when, from the occasional superposition of other beds, 

 or the absence of portions from the effects of waste, 

 they cannot be absolutely traced, still, their former 

 extent, or even their present, is easily inferred by the 

 well-known rules of geology, namely, the comparison 

 of characters, relative position, and angles of inclina- 

 tion. But the unstratified rocks are rarely found 

 occupying extensive continuous spaces on the surface; 

 being, on the contrary, interrupted by the interven- 

 tion of portions of strata, or else appearing in distinct, 

 and often in small eminences scattered through a 

 district, of which the fundamental part is stratified. 

 The minuteness of these is often indeed very remark- 

 able ; since, in Scotland, where both these classes of 

 rocks abound, it is not unusual to find a portion of 

 granite, of a very few yards in diameter, separated by 

 many miles from any other of the same rock ; while 

 the same is frequently observed with respect to 

 trap. 



The cause, in these two cases, is, however, dif- 

 ferent in its nature. By examining sections and 

 tracing the condition of the insulated masses of granite 

 and of the conterminous strata, it is discovered that this 



