WITH AGRICULTURE AND PLANTING. 477 



a crystalline, granular, or slaty texture. The mutual 

 adhesion of the heterogeneous parts, of which they con- 

 sist, being, in general, inconsiderable, they are easily 

 broken down by mechanical means, and thus contribute in 

 a high degree to the formation of productive soil. The 

 felspar contained in these rocks, on account of the chemi- 

 cal decomposition which it readily undergoes, has a great 

 effect not only upon the quantity, but also the fertility 

 of the soil produced. The quartz, on the contrary, as 

 well as the mica and hornblende, long resist chemical de- 

 composition; they are, however, useful in this respect, that 

 the argillaceous soil arising from the felspar, has its tena- 

 city diminished ; and is consequently rendered better 

 adapted for vegetation, by being intermixed with them. 

 Granite and gneiss, of all truly granular crystalline rocks, 

 afford the deepest and most fertile soil, aptly compound- 

 ed of different substances, sufficiently loose in its aggrega- 

 tion, and capable of retaining the necessary moisture. 

 Soil arising from the disintegration of granite is unfa- 

 vourable to vegetation only, where the rock abounds much 

 in quartz, and where the superfluous water cannot run 

 off, and so gives rise to marshes, which produce only ve- 

 getables of inferior quality ; of which we have examples 

 in the granite districts of Aberdeen. In such places as 

 these, peat is easily generated, which, although of great 

 use, is yet much less advantageous than wood. Syenite^ 

 which abounds much in hornblende, is inferior to gra- 

 nite, with respect to the production of fertile soil ; and 

 primitive greenstone^ which resists disintegration and de- 

 composition in the highest degreej occupies the last place 

 in this class. In the series of slaty crystalline rocks, mica- 

 slate is next to gneiss : but on account of the small pro- 



