Connvall, Devon, and West Somerset. 33 



Trengwainton, near Penzance, from Chyoonto Mousehole) famous 

 for producing two crops of potatoes in the year), in the vicinity 

 of Penryn, at St. Michael's Mount, and at Pendarves. This may 

 no doubt be true in many cases, but it is far from being a general 

 fact. Indeed we can readily perceive that the benefit of such a 

 mixture must depend upon the mineralogical character of both 

 rocks at their junction ; now as these vary materially, so we 

 should consider would be the agricultural value of the effect pro- 

 duced. Those parts of the Hensborough or St, Austell granite 

 which contain much schorl, as a large portion of the western side 

 does, are far from fertile, and the altered slate rocks in contact 

 with them are often sterile, as may be seen on Fatwork Hill. 

 The like may be seen on Castle-an-Dinas and Belovely Beacon. 

 A large proportion of the skirts of Dartmoor is poor land, espe- 

 cially those parts which come into contact with the carbona- 

 ceous series. In those situations where the decomposed drift 

 from the trappean rocks has mingled with the soil, as near 

 Tavistock, Okehampton, Bridford, Cristow, and Hennock, the 

 soil is much improved, and being close to the granite, so that the 

 disintegrated portions of the latter are occasionally washed down 

 and mingled with it at the same time, the opinion above noticed 

 might appear to be more borne out in those situations than it 

 really is. As the granite is more constant in its mineralogical 

 character than the rocks which come into contact with it, and 

 generally become more or less altered, often a sufficient reason in 

 itself for their decomposed portions to be more fertile than they 

 might otherwise be, probably the benefit that may be derived 

 from any mixture of the growan and schistose soils mainly depends 

 upon the mineral composition of the slates near their junction with 

 the granite. 



The alluvial lands, though generally fertile, vary, as might be 

 expected, according to the mineralogical composition of the rocks 

 from which they have been chiefly derived, as may be seen by 

 comparing the flat lands bordering the rivers in numerous places. 

 Let, for example, the alluvial land on the banks of the Culm, 

 traversing red marl and sandstone, be compared with that between 

 Sheep wash and Monks Okehampton on the banks of the Tor- 

 ridge, flowing among the carbonaceous series, or with many 

 among the granitic districts, and there will be little difficulty in 

 perceiving the superior fertility of the former. Numerous com- 

 pounds would appear to be effected, in some places producing 

 better soils than each rock separately affords : of this, probably, 

 the alluvial lands among the limestones and slates of the grau- 

 wacke may often be taken as examples. Wherever the rivers 

 have held their courses amid easily decomposed trappean rocks, 

 the usual grauwacke compounds appear much improved. In- 



VOL. III. D 



