264 EXTENT AND SOIL OF THK TRAP-r.OCKS. 



Stone, chiefly by the darkness of its colour, and by the minuteness of the 

 particles of which it is composed, which, in general, cannot be distin- 

 guished by the naked eye. 



Zeolite is a generic term applied to a great number of mineral specieg 

 which occur in the basalts, and often intermixed with the green-stone 

 rocks. They differ from felspar in their greater solubility in acids, and 

 by generally containing limey where the latter contains potash or soda. 



It may be stated, indeed, as the most important agricultural distinc- 

 tion, between the granitic and the true* trap-rocks, that the latter abound 

 in lime, while in the former, it is often entirely absent. If in a green- 

 stone only one-fourth of its weight consist of augite, every 20 tons of the 

 rock may contain one ton of lime. If in a basalt the augite and zeolite 

 amount to only two-thirds of its weight, every nine tons may contain a 

 ton of lime. The practical farmer cannot fail to conclude that a soil 

 f()rmed from such rocks must possess very different agricultural capabil- 

 ities from the soils we have already described as being formed from the 

 decomposition of the granites. 



3°. Serpentine is a greenish yellow mineral, consisting of silica in 

 combination with magnesia and a little iron, and occasionally a few 

 pounds in the hundred of lime or alumina. The distinguishing ingredi- 

 ent is the magnesia, which generally approaches to 40 per cent, of the 

 whole weight of the mineral. Rocks of serpentine are generally mixed 

 with magnetic iron ore, and with portions of other minerals in greater 

 or less abundance. 



Extent of the trap rocks in the British Isles. — The serpentine rock oc- 

 curs to any extent anly in Cornwall, about the Lizard Point, where it 

 covers an area of 50 square miles. The green-stones and basalts are 

 only met with here and there in small patches, until we get so far north 

 as the Cheviot Hills, which consist of these and other varieties of trap. 

 It is in the low country of Scotland, however, intermixed with and sur- 

 rounding the great coal district of that part of the island, that the greatest 

 breadth of trap is seen. It there stretches across the island in a south- 

 west direction, and in detached masses, from the Friths of Tay and 

 Forth to the island of Arran, covering an area of 800 or 1000 square 

 miles. In the prolongation of the same line it re-appears in the north- 

 east of Ireland, and extends over the whole of the county of Antrim and 

 u small part of Londonderry and Armagh. In the most northerly portion 

 of this tract the well-known columnar basalt of the Giants' Cause^yay 

 occurs. On the west coast of Scotland the trap rocks cover nearly the 

 whole of the islands of Mull and of Skye — to the west of the former of 

 which islands lies Staflfa with its celebrated basaltic caves. 



Soil of the trap rocks. — The soil of the serpentine rocks at th« Lizard 

 is far from fertile, retaining the water and thus forming swamps and 

 marshes. Even where a natural drainage exists it rarely^pioduces good 

 grass, or average crops of corn. It is remarkable for growing a pecu- 

 liar, very beautiful heath — erica vagans — which so strictly limits itself 

 to the serpentine soil as distinctly to mark the boundary by which the 

 serpentine is separated from other rocks (De La Beche). From the 



* Serpentine is not generally included among the trtie trap rocks : It is included among 

 tliem here as it often is by geologists, because in many places, as at the Lizard, it occurs 

 along with true green-stone 



