HEATHS AXD BOGS ON THE GNEISS ROCKS. 255 



less heath or to widely extended bogs. Yet the slate rocks themselves, especi- 

 ally when they happen to be calcareous, are capable of producing fertile soils. 

 Such are found in the valleys, on the hill sides, and by the margins of the lakes 

 that are often met with in the slate districts. More extensive stripes or bands 

 of such productive land occur also at lower levels, as in the north of Devon, and 

 in the south of Cornwall. In the latter county, the soils on the horTiblende slate 

 (which lies near the bottom of the slate seriesjare extremely fertile, exhibiting a 

 striking contrast with those which are formed from the neighbouring Serpentine 

 rocks, that extend over a large area immediately north of the Lizard (see. p. 265.) 



Where *the clay-slate soils occur, therefore, however cold and stiff they may 

 be, a favourable climate, drainage, if necessary, and lime, either naturally pre- 

 sent, or artificially added, appear to be the first requisites to insure fertility. 



The mode in which these rocks lie, or the degree of inclination which the 

 beds exhibit, exercises an important influence upon the agricultural character 

 of the soils that rest upon them. In the diagram inserted in page 238, the 

 rocks (A) represent the highly inclined, often nearly vertical position, in which 

 the slate rocks are most frequently found. The soil formed from the¥*i must, 

 therefore, rest on the thin edges of the beds. Thus it happens in many lo- 

 calities that the rains carry down the soluble parts of the soil and of the manure 

 within the partings of the slates — and hence the lands are hungry and unprofit- 

 able to work. 



On the slopes of the clay slate hills of the Cambrian and Silurian system?, 

 flourish the vineyards of the middle Rhine, the Moselle, and the Ahr. 



H. — Mica-Slate and Gneiss Systems. 

 21°. Mica-Slale, Gneiss Rock. The upper of these formations con- 



sists of thin undulating layers of rock, 

 consisting chiefly of quartz and mica, 

 alternating occasionally with green 

 (chlorite) slates, common clay-slates, 

 quartz rock and hard crystalline lime- 

 stones. The gneiss is a hard and 

 solid rock of a similar nature, consist- 

 ing of many thin layers distinctly vi- 

 sible, but firmly cemented, and as it 

 were half-melted together. 

 Extent, — Two-thirds of Scotland, comprehending nearly the whole country 

 north and west of the Grampians, consist of these rocks. In England there 

 is only a small patch of mica slate about Bolt Head and Start Point in South 

 Devon, and a somewhat larger in Anglesey ; but in Ireland, nearly the whole 

 of the counties of Donegal and Londonderry on the north, and a large portion 

 of Mayo, Connaught, and Gal way, on the west, are covered by rocks "belonging 

 to the mica slate system. 



Soils. — These rocks are, in general, harder still than those of the Cambrian 

 system, and still more impervious to water, when not highly inclined. They 

 crumble slowly, therefore, and imperfectly, and hence are covered with thin 

 soils, on which, where good natural drainage exists, a coarse herbage springs, 

 and from which an occasional crop of corn may be reaped — but on which, where 

 the water becomes stagnant, extensive heaths and bogs prevail. That they 

 contain, when perfectly decomposed and mellowed, the materials of a fertile soil, 

 is shown by the richness of many little patches of land, that occur m tiie shel- 

 tered valleys of the Highlands of Scotland, and by the margins of^ its jnany 

 lakes. In general, however, the mica-slate and gneiss country is s6 effltated 

 that not only does an ungenial climate assist its natural unproductiveness, but 

 the frequent rains and rapid flowing rivers bear down to the bottoms of the yal- 

 lies or forward to the sea, much of the fin-r matter produced by the decay of the 

 rocks, — leaving only a poor, thin, sari !y soil i),^liii d. 



