§ 25. ISLE OF STAFFA. 851 



been articulated together, as in the basaltic columns of the 

 Giants' Causeway (Lign. 190). 



Proofs of the correctness of this inference are afforded 

 by the occurrence of a spheroidal nucleus enveloped by 

 a polyhedral block of basalt ; and from the fact, that when 

 this rock is not divided into regular prismatic columns, 

 it often forms laminated spheroids, which, varying in size, 

 constitute by aggregation extensive masses. The position 

 of the columns presents every variety from the perpendicular 

 to the horizontal ; this has arisen from corresponding dif- 

 ferences in the direction of the cooling surfaces, for the 

 prisms are found to be always at right angles with the 

 surface of refrigeration ; the horizontal, inclined, and curved 

 columns of basalt, which occur in the Isle of Staffa, and 

 elsewhere, have originated from this cause. 



Lign. 191. — The Isle of Staffa. 



25. Isle of Staffa • Fingal's Cave.— Many of the 

 Hebrides, or Western Isles of Scotland, are almost wholly 

 composed of trap rocks. Of these Staffa* is the most 

 celebrated, on account of a deep chasm or recess situated in 

 a magnificent group of vertical columnar basalt, and which 

 * Staffa, a Norse term signifying a staff or column. 



VOL. II. 3 K 



