Rocks with PlantsW^ 337 



that the chief part of the mineral ingredients of soil is not 

 derived from the rock beneath, but has been transported from 

 some distance. The decay of plants is constantly tending to 

 increase the foreign matters on the earth's surface, and to 

 separate the vegetable kingdom still farther from the rocks. 

 This vegetable soil must abound chiefly in climates favourable 

 to the production of luxuriant vegetation ; but it may deserve 

 notice, that in Orkney, the Western Isles, and even in the most 

 northerly parts of Scotland, there is often found a thick and 

 dense covering of peat, which must prevent any chance of 

 native plants being influenced by the rocks. Further, it will 

 appear, that, even when the soil has not been transported, the 

 following grave objections occur to the connection here dis- 

 cussed. ° " ou *n9DianoD e r iatioda a rf^uorLr t T £*ioJ3ctaite 



Different specimens of the same rock differ materially in 

 respect of the circumstances most likely to influence vege- 

 tables ; namely, in the nature and proportion of the consti- 

 tuents, together with the degree of tendency to pass into the 

 condition of soil. For instance, the granite of Aberdeenshire 

 has almost every possible variety in composition and hard- 

 ness. Sometimes it resists the weather in a complete manner; 

 while, on the other hand, it frequently furnishes an abundant 

 soil. It is worth adding, that, in the parish of Strathdon, I have 

 noticed specimens of this rock retaining all its usual appearance, 

 which proved to be masses of almost loose earth extending to a 

 great depth. Gneiss, too, is subject to frequent and material 

 differences. Dr. Macculloch, in his late work on geology, 

 has particularly noticed the bareness of many of the Scottish 

 gneiss isles ; whereas, in Guernsey and Aberdeenshire, a deep 

 and good soil is often derived from the same rock. Passing 

 to the secondary rocks, we may find basalt, greenstone, amyg- 

 daloid, and others, often varying in structure ; each being 

 occasionally of the most obdurate description, though more 

 commonly producing a copious and fertile soil. I shall only 

 further specify sandstone, a rock which is important in the 

 present view, as it occupies large tracts of country in many 

 parts of the world. It is clear that there could be no unvary- 

 ing relation between this rock and the vegetables which grow 

 over it, as it is characterised in no degree by the nature of its 

 ingredients, but by circumstances altogether of another kind ; 

 the rock differing entirely from itself in different instances, 

 being siliceous, calcareous, or aluminous, according to cir- 

 cumstances. So great are these differences, that sandstone is 

 sometimes nearly allied to quartz rock ; while, occasionally, 

 it is with difficulty distinguished from limestone. Not only 

 does the same species often exhibit material varieties, but, on 



Vol. VI. — No. 34. z 



