596 SKY. GEOLOGY. TRAP VEINS. 



guislied. I have already slightly mentioned them in the 

 general description of this coast, but must now be more par- 

 ticular, and, to give an idea of their general appearance, 

 have subjoined what must rather be considered as a plan 

 than a drawing, since the extent and nature of the subject 

 admit of nothing else.* I formerly noticed their extraor- 

 dinary numbers, and may now add that in consequence of 

 their frequency, they in some places nearly equal, when 

 collectively measured, the stratified rock through which 

 they pass. Six or eight sometimes occur in the space 

 of fifty yards, their collective dimensions being apparently 

 not less than sixty or seventy feet. This remark is not 

 mere matter of curiosity, since, as will presently be seen, 

 it leads to geological inferences not unworthy of regard. 

 Their direction is almost invariably either vertical or slightly 

 inclined from the perpendicular, and they present therefore 

 a nearly regular parallelism along the coast. Each vein is 

 of equal size throughout its visible course, and without rami- 

 fications ; and although they vary from five feet to twenty 

 in breadth, they more commonly do not exceed ten. It 

 is well known that the permanence of trap veins is some- 

 times less, at others greater than that of the surrounding 

 rocks. Hence they sometimes project like walls, while 

 at others their ruin produces fissures or caves. This latter 

 effect has taken place here, marking their great tendency 

 to decomposition, since the including rocks do not seem 

 to be of a very durable nature. The depth to which they 

 have been excavated is often very considerable : in the 

 case of the Spar cave, including the external fissure, it 

 cannot be less than 250 feet. In consequence of this 

 wasting, the intermediate cliffs which remain, have, as 1 

 have noticed in the general description, the appearance 

 of the ends of walls ; and as they sometimes yield and 

 fall away behind, they present in such cases the appear- 

 ance of insulated square pillars of masonry ; the resemblance 



* Plate XVI. fig. 1. 



