448 CANNA. GEOLOGY. 



THE rocks of Canna, however various in their aspects, 

 all appertain to the trap family. It is fruitless to describe 

 these varieties, and impossible to specify exactly the order 

 of their arrangement, since it is no where constant. The 

 description of the alternations in one portion of the island 

 would not apply to another, and as no useful informa- 

 tion could be conveyed by a minuteness of detail in this 

 respect, nor any geological conclusions drawn which 

 would not equally follow from a description more general, 

 the reader will be satisfied with the latter. 



Basalt* both columnar and amorphous, several varieties 

 of greenstone, trap porphyry, and amygdaloids, are seen 

 variously disposed throughout the island. The amyg- 

 daloids, like those of Sky, contain various minerals of 

 the zeolite family, among which analcime is the most 

 conspicuous. Considerable crystallizations of several of 

 the forms of stilbite and of filamentous nadelstein are 

 also to be seen, but these are more rare ; while the 

 specimens are seldom so accessible as to add to the 

 acquisitions of a collector ; those that occur being also 

 injured by the weather or by their fall from the high 

 "cliffs. 



The columnar forms are most prevalent on the southern 

 side of the island, where many ranges may be observed 

 rising in terraces from the shore to the uppermost level ; 

 none of them attaining a greater height than perhaps twenty 

 feet, and some not exceeding three or four in altitude. 

 Many of the columns are as regular in their angles and 

 joints as the best formed of those which occur in the 

 Western islands, while others gradually pass into amor- 

 phous basalt. 



Near the ancient castle, illustrated by an engraving in 

 Pennant's work, the singular situation of which has attracted 



* O 



the attention of all travellers, the approach to a colum- 

 nar character is displayed in a very striking manner in 



* Plate XIX. fig. 2. 



