WITH AGRICULTURE AND PLANTING. 463 



For this reason, mountains consisting of quartz or por- 

 phyry, for example, very frequently present surfaces des- 

 titute of vegetation ; while, on the other hand, those of 

 granite, slate or sandstone, are more frequently adapted 

 for agriculture and planting. In the northern parts of 

 Scotland, quartz rocks, destitute of all vegetation, rise 

 in the midst of mountains covered with gramineous plants, 

 and sometimes wood. In the most fertile part of the 

 south of Norway porphyritic mountains rise from a cal- 

 careous and schistose base, with lofty, rugged, and bare 

 cliffs. In the southern parts of the Tyrol the rocky ste- 

 rility of the abrupt and lofty porphyritic mountains pre- 

 sents a striking contrast to the fertility of the neighbour- 

 ing limestone mountains, which are covered with vines, 

 walnuts and chesnuts. 



The surface of the solid strata of the earth has also an 

 indirect influence upon the cultivation of plants, in so 

 far as the water which the vegetable mould acquires 

 from the atmosphere, is retained in the soil, or is drawn 

 off by the subjacent rock. Different rocks produce very 

 different effects in this respect, depending as well upon 

 their constitution as their structure. The component 

 parts of rocks imbibe water in different modes and de- 

 grees ; and different sorts of rocks not only attract water 

 with different celerity, but also imbibe different quanti- 

 ties of it. The latter difference depends chiefly upon 

 the various substances of which rocks are composed, 

 partly, also, upon their porosity. Siliceous rocks attract 

 water in the lowest degree, argillaceous ones in the high- 

 est, and calcareous rocks appear to have an intermediate 

 action in this respect. Compact and granular crystal- 

 line rocks attract water in a smaller degree, and more 

 slowly ; friable or crumbled rocks imbibe it in greater 



