262 GRANITE ROCKS OF GRF.AT BRITAIN' AND IRKLAND. 



fects, — and will encourage liim to persevere in a course of treatment 

 which, while suggested by theory, is confirmed also by practice. 



Extent of granitic rocks in Great Britain and Ireland. — In England, 

 the only extensive tracts of granite occur in Cornwall and Devon, pre- 

 senting themselves here and there in isolated patches from the Scilly 

 Isles and the Land's End to Dartmoor in South Devon. In the latter 

 locality, the granite rocks cover an area of about 400 square miles. Pro- 

 ceeding northward, various small out-bursts* of granite appear in the 

 Isle of Anglesey, in Westmoreland, and in Cumberland, and north of 

 the Solway, in Kirkcudbright, it extends over 150 or 200 square miles; 

 — but it is at the Grampian Hills that these rocks begin to be most ex- 

 tensively developed. With the exception, indeed, of the patches of old 

 red sandstone already noticed, nearly the whole of Scotland, north of the 

 Grampians — and of the western islands, excluding Skye and Mull, con- 

 sists of granitic rocks. 



In Ireland, a range of granite (the Wicklow) mountains runs south by 

 west from Dublin to near New Ross — the same rock forms a consider- 

 able portion of the mountainous districts in the north-west of Donegal, and 

 in the south of Galway — covers a less extensive area in Armagh, and pre- 

 sents itself in the form of an isolated patch in the county of Cavan. 



Soils of the granitic rocks. — From what has been already stated in re- 

 gard to the composition of granite, it is clear from theory that no gene- 

 rally uniform quality of soil can be expected to result from its decompo- 

 sition, and this deduction is confirmed by practical observation. Where 

 quartz is more abundant, or where the clay is washed out, the soil is 

 poor, hungry, and unfruitful — such, generally, is its character on the 

 more exposed slopes of the hills in the Western Isles, and in the north 

 of Scotland. — [Macdonald's Agricultural Survey of the Hebrides, p. 26.] 

 In the hollows and levels, where natural drainage exists, stiff clay soils 

 prevail, which are often cold and unfruitful, but are capable of amelio- 

 ration where the depth of earth is sufficient, by draining and abundant 

 liming or marling. Where there is no natural drainage, vegetable mat- 

 ter accumulates, as we have seen to be the case on the surface of all im- 

 pervious rocks — and bogs are formed. In the north of Scotland, and in 

 Ireland, and in the high lands of Dartmoor (Devon), these are every- 

 where seen in such localities, and it is'said that two-thirds of the He- 

 brides are covered with peat bogs more or less reclaimable. 



In Cornwall and Devon, the granitic soils {growan soils, as they are 

 there called) are observed to be more productive as the hills diminish 

 in height. Thus Dartmoor is covered only with heath, coarse grass, 

 and peat ; while in the Scilly Isles the growan land produces good crops 

 of wheat, potatoes, barley, and grass ; and the same is observed at 

 Moreton Hampstead, in Devon, where tolerable crops of barley are grown, 

 and potatoes, which are highly esteemed in the Exeter market (De La 

 Beche). No doubt the climate has something to do with these differ- 

 ences ; but the less the elevation, and the consequent washing of the 

 rains, the more of the clay will remain mixed with the siliceous sand ; 



* This expression is in some measure theoretical, and implies — what is the generally re« 

 ceived opinion— that the granite rocks were forced up from beneath in a fluid state, like the 

 lavas of existing volcanoes— that they, as well as the trap rocks, are, in short, only lavas of 

 a more ancient date (see p. 237). 



