58 Dr A. Murray ati the hifluence of Rocks 



and the rocks bearing those names, because in different in- 

 stances each differs entirely from itself. 



Not only does the same rock differ materially ; but, on the 

 other hand, rocks whose geological and mineralogical characters 

 are dissimilar, may furnish a soil essentially the same. Thus, 

 the following branches of the primitive series : granite, gneiss, 

 mica- slate, clay-slate, when converted into soil, all usually give 

 rise to a sandy clay ; and, with respect to the secondary trap- 

 rocks, they run into one another by insensible gradations, and 

 each of them will probably produce a soil similar to that from 

 any one of the rest. 



In short, the varied forms of a particular rock may differ 

 from one another, in respect of the circumstances likely to influ- 

 ence vegetation, more than certain rocks do from others, which 

 are reckoned different in species, — a consideration which must 

 occasion serious practical difficulties to him who attempts to 

 connect plants with rocks. It might no doubt be alleged that 

 this objection is more apparent than real, and that we must not 

 class rocks together, which have little or no similarity but in 

 name ; but wnth a reference to the present object must reckon 

 as the same those rocks only which have the same tendency to 

 crumble down, or to be chemically decomposed, and which 

 form soil of a similar description. Even with this modification, 

 ■which cannot be adopted in practice, the doctrine here com- 

 bated will be found to have no solid foundation. 



In the first place, it cannot be denied that at the present day 

 rocks usually lie at so great a depth, that the roots of most ve- 

 getables cannot come into contact with them ; and it hence be- 

 comes probable that in general rocks have little influence upon 

 the vegetables growing over them, — at least, except when they 

 contribute materially to the superincumbent soil. Now, I be- 

 lieve, it will be admitted by all who have attended to the sub- 

 ject, that when an opportunity is afforded for making the ob- 

 servation, it is frequently apparent that the chief part of the 

 mineral ingredients of the soil is not derived from the subjacent 

 rock, but has been transported from some distance. These 

 changes of situation, I scarcely need to say, are ascribed to the 

 agency of water in various ways, — that is, rain and ice ; to rivers 

 particularly in a state of flood, from the deposites which take 



