80 Dr. Mac 0\jl-locvC^ Sketch of the 



menslons could not be less than 60 or 70 feet. This remark is not 

 mere matter of curiosity, it leads to geological inferences not un- 

 worthy of regard, as will be presently seen. Their direction is 

 almost invariably either vertical or slightly inclined from the plumb, 

 and they present therefore a perpetual parallelism along the coast. 

 They are equal throughout, and never ramify, and although they 

 vary from 5 feet to 20 in breadth, they more commonly are of a 

 dimension not exceeding 10 feet. It is well known that the perma- 

 nence of trap veins is sometimes less, sometimes 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 a great tendency to decompo- 

 sition in these veins, since the including rocks do not seem to be of 

 a very durable nature. The depth to which they have been exca- 

 vated is often very considerable : in the case, of the spar cave inclu- 

 ding the external fissure, it cannot be less than 250 feet. In conse- 

 quence of this wasting it happens that the intermediate cliffs which 

 remain, have, as I have noticed in the general description, the ap- 

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

 fall away behind, in such cases they present the appearance of in- 

 sulated square pillars of masonry, the resemblance being rendered 

 perfect by the channelled marks of the strata formerly described. 

 These veins are often stratified, or more properly speaking, lami- 

 nated in the direction of their length. They are generally formed 

 of a bluish black basalt ; at times they are porphyritic, or vary in 

 other ways, which it is unnecessary to describe. I observed in one 

 nodules of prehnite, the only occasion on which I have found that 

 mineral in veins of trap. In another I found a second vein, holding 

 a serpentine course through the first in a somev/hat parallel direction, 

 and readily distinguished by being formed of a much more black 



