OVERLYING AND TRAP ROCKS. 117 



such circumstances. I cannot doubt that some of 

 these were originally, eruptions of volcanic " ashes," 

 or volcanic mud ; and they confirm that identity of 

 origin between volcanic and trap rocks so often here 

 inculcated. And, from what I can observe, the other 

 kind appear to have been formed out of casual and 

 local fragments, produced in the usual manner from 

 antient traps, and subsequently overwhelmed by new 

 eruptions. At home, this case is evident in Canna ; 

 and if I cannot find it distinctly recorded by others, 

 there is an analogy to support it in that case of granite 

 already quoted at Predazzb. 



The great features of this whole family present ex- 

 amples of every species of outline found among other 

 rocks, with some which seem peculiar to themselves. 

 And if sometimes regulated by the nature of the dif- 

 ferent leading varieties, considerable differences in the 

 outline of a country sometimes occur, where the 

 rocks are the same ; and the reverse. 



It is not unusual to find a land of low and tame 

 undulations, consisting solely of claystone, porphyry, 

 and greenstone, while the same rocks produce compli- 

 cated groups, or ridges, or single mountains, of consi- 

 derable elevation and of distinct rounded, or conoidal 

 forms, rarely exhibiting any other abrupt faces than 

 those produced by the casual position and flow of 

 water-courses ; the causes of these tame forms con- 

 sisting in the yielding nature of the materials, and in 

 the ready disintegration which the rocks undergo. 

 Thus, by the gradual descent of the fragments, the 

 original asperities are softened and the cavities filled 

 up ; while, the whole progress of the change may 

 often be traced, from the sharp -and projecting face or 

 pinnacle, to the even and gently swelling declivity. 

 Where inimical to vegetation, or when the descent of 



