142 BASALT OF STAFFA AND EGG. COLUMNAR TRAP. 



The basaltic columns of ihe Island of StafRi are too well known to 

 require a description ; but, according to Dr. MacCulloch, the columns 

 which form the lofty promontory called the Scuire of Egg, another of 

 the Hebrides, exceed in grandeur and in picturesque effect those of 

 Staffa : they are formed of black pitchstone, containing crystals of glos- 

 sy felspar. " The promontory rests on a bed of compact grey lime- 

 stone, approaching to a stone marie. This bed, which is three or four 

 feet thick rests on a still lower bed of hard reddish stone. Masses of 

 bituminized wood, penetrated with carbonate of lime, are found in the 

 marie stratum not at all flattened. Portions of trunks of trees, retain- 

 ing their original shape, but petrified (silicified,) are found in the same 

 stratum ; the rifts are filled with chalcedony, approaching in aspect 

 to semi-opal. The columns on this island are both perpendicular and 

 inclined, and some of them are bent or curved." 



In various parts of Scotland and the Hebrides, the tendency to a 

 columnar arrangement in the basaltic rocks may be distinctly seen : 

 it is obscurely developed in the basalt of Arthur's Seat near Edin- 

 burgh. The basalt of this hill appears identical with some of the 

 volcanic mountains I examined in Auvergne, particularly near the 

 summit of Montadoux, a mountain near Cleremont. 



In England the columnar structure of some of the basaltic and 

 trap rocks is observable in the northern counties, particularly on the 

 banks of the river Tees, and at Swarthfell near Ulswater. In some 

 of the basaltic hills near Dudley, the columnar structure is developed 

 but the columns are not separated and well defined. Prismatic 

 blocks of sienite, are scattered over a hill of sienite called Markfield 

 Knowl, at Charnwood Forest in Leicestershire. 



Columns of porphyritic trap or greenstone occur in groups, on 

 the northern side of Cader Idris in Merionethshire. One of these 

 columnar groups is represented Plate V. fig. I.; the outline of the 

 columns was taken with a camera lucida by Henry Strutt, Esq. of 

 Derby, and cannot fail to be correct; the figure is introduced, to 

 show the relative magnitude of the columns. Rocks of trap and ba- 

 salt, both in solid beds, and also arranged in columns like those of 

 Staffa, were observed by Sir G. Mackenzie, on the coast of Iceland 

 and also in the interior ; the lower parts of the beds and columns 

 contained scorise and slags, and empty cavities. A successive range 

 of beds of basalt was also observed alternating with beds of tufa, the 

 lower parts of which presented the same appearance of the action 

 of fire. 



From the situation of these rocks, and from the existence of sub- 

 marine volcanoes near Iceland, Sir G. Mackenzie conceives that 

 these beds of basalt were formed under the sea by the ejection of 

 lava, which, flowing over the moist submarine ground, would confine 

 a portion of water beneath the melted mass : this water would be 

 converted into elastic vapour, or steam, which would endeavor to ex- 

 pand : but where the superincumbent pressure of the ocean, or the 



