652 GEOLOGY. 



common appearance exhibited in the carboniferous system, that of the coal being reduced 

 to a cinder, and converted into coke, in contact with the trap dykes, which intersect the 

 strata. In general, also, strata of every kind require a greater degree of induration in 

 contiguity with trap rocks, loose grits passing into compact quartz, and shales into flinty 

 slates, evidencing the action of a high temperature upon them. The engraving represents 



Three Positions of Trap. 



a section of the sandstone strata, in the cliffs on the east coast of the Isle of Skye, which 

 exhibits all the three modes of position assumed by the trap rocks, in the same mass of 

 trap. To the left a large mass appears overlying the strata ; from this a horizontal bed 

 runs conformably through the midst of the strata, ultimately becoming divided into 

 beds, three smaller interstratifying the sandstone ; and from the horizontal bed the trap 

 diverges in dykes, or walls. 



The following principal members of the trap family are selected for description and 

 remark. 



Greenstones. These are popularly called whinstones. They are the most common 

 species of trap, and are ordinarily composed of hornblende and felspar in nearly equal 

 proportions, and less commonly of augite and felspar. When the hornblende preponderates, 

 the rock so formed usually receives the name of hornblende rock. When the grains of 

 felspar and hornblende are quite coarse, the result is sienitic greenstone, which is 

 frequently found in combination with quartz, and then passes into sienitic granite. But 

 the varieties are numerous, and are differently distinguished in the geological nomenclature. 

 The columnar or prismatic structure is not unfrequently displayed by greenstone, some- 

 times rudely, as at Corygills in the Isle of Arran, but in other cases, in a very definite 

 and magnificent manner. It is found in almost every region of the globe, and is 

 abundantly distributed through the coal strata of Great Britain. In the Corstorphine 

 Hills near Edinburgh, the sandstone is capped with greenstone, a hundred. and fifty feet 

 thick, composed of felspar and hornblende, disposed in beautiful stellar concretions, of a 

 yellowish-green hue. It occurs, in the same locality, in the Salisbury Crags, and in Arthur's 

 Seat. 



Basalts. This variety of that mass of melted rock which has been ejected at different 

 periods from the interior regions of the globe is substantially identical with greenstone in 

 mineral composition, consisting of hornblende and felspar, or augite and felspar ; but these 

 ingredients are more intimately combined and finely granular. A considerable per-centage 

 of oxide of iron is also found in basalt, and distinct grains of olivine, a variety of 

 chrysolite. The rock has a compact and uniform texture, and a greenish or iron-grey 

 colour, approaching to black. It occurs in very extensive masses, constituting many 

 thousand square miles of the surface of the Deccan in India. Basalt forms dykes and 

 horizontal beds, in both of which it very frequently displays the columnar and prismatic 

 structure, which has rendered it an object of great popular interest. Viewed from above, 

 the tops of a number of basaltic columns appear like a pavement artificially composed of 

 hexagonal or polygonal pieces of stone, nicely fitted into each other, as \&fig. 1. Viewed 

 laterally from a distance, a range may readily be mistaken for a pile of buildings the 

 frequent appearance of the basalt which rises up suddenly from the plains of the Deccan. 

 Viewed in the same direction, in situations where the columns are jointed, and have been 

 exposed to the destructive action of the ocean, the mass resembles a building in ruins, as 



