PLUTONIC OR IGNEOUS ROCKS. 



653 



in fig. 2 Fig. 3. represents a specimen of horizontal basalt. Two beautiful examples of 

 basaltic columns are at Staffa, one of the Hebrides, and in the county of Antrim on the 



^fe^t-i 



Ffr.1 



.2. 



north coast of Ireland, where they form the celebrated 

 Giant's Causeway. The latter is a platform running out 

 into the sea, composed of the most compact and homogeneous variety of basalt, which 

 is more or less sonorous when struck with a hard substance. The columns forming the 

 causeway rarely exceed a foot in breadth, and thirty feet in height. They are chiefly 

 hexagonal, but polygons, of five, seven, and eight sides, are of frequent occurrence ; and 

 there is one example of a triangular prism. It is not, however, at the Causeway that the 

 largest-sized columns are found. Fair-head, the highest promontory on the coast, rising 

 five hundred and fifty feet above the level of the sea, is entirely composed of them, some 

 of which, according to the accurate measurement made during the recent Ordnance trigo- 

 nometrical survey of Ireland, are 317 feet in 

 height, the sides of these enormous prisms occa- 

 sionally measuring five feet. The vicinity of 

 the Causeway affords numerous proofs of the 

 basalt having been erupted in a state of fusion 

 from heat, in the effect produced upon the 

 secondary strata in contact with it, the old red 

 sandstone being changed into hornstone, the 

 clay-slate into flinty slate, the coal into coke, 

 and the chalk into granular marble. The ac- 

 companying illustration represents the basalt 

 rocks at Bertrich-Baden, called the " Grotte 

 des Fromages," from its resemblance to a pile 

 of cheeses. 



Wacke and claystone are soft varieties of basalt, having an earthy admixture under 

 different degrees of induration. Clinkstone is a basalt in which the felspar greatly 

 predominates, and the texture becomes very compact and hard, so as not to be easily 

 scratched with a knife. The rock is of a massive structure, not crystallised, and of a 

 greenish or iron-grey colour. It derives its name from yielding a metallic sound upon 

 being struck, and closely resembles the basalt of the Causeway, sometimes exhibiting the 

 same columnar structure, as in Swarthfell in Cumberland, where it is defined upon a 

 magnificent scale. Pitchstone, another basalt, containing a portion of bitumen, is of a 

 glassy green colour, resembling obsidian or volcanic glass. It occurs in veins in the Isle 

 of Arran, and in the county Down in Ireland ; and is found extensively in the hills 

 around the valleys of Tribioch near Messein in Saxony. The lofty promontory called 

 the Scuir of Egg, in the island of that name, one of the Hebrides, is a gigantic pitchstone 

 vein, which seems to have been left in the destruction of the surrounding rocks. The 

 promontory rises 1339 feet above the sea; and the pitchstone, which is intensely black, 

 with crystals of glassy felspar, forming a substance of great beauty, is arranged in columns, 



Grotte des Fromages. 



