52 GEOLOGY OF THE LOTHIANS. 



ly below the trap lies the sandstone, which, in the vicinity 

 of the greenstone, becomes quartzose, and also exhibits in 

 several places various undulations. In one place there is a 

 fine display of one of those veins, which are to be consi- 

 dered as forming the points of eruption. It is about 4 

 feet in breadth, and is vertical. (PL III. Fig. 1.) From 

 this vein a lateral expansion takes its origin, running into 

 the strata parallel to their direction ; at its origin it is about 

 2 feet broad, and after running for 10 feet a slightly tortu- 

 ous course, it gradually thins out. On the principal vein 

 arriving at the surface of the sandstone it flows over it, and 

 forms the overlying trap-rock of St Leonard's Hill, and 

 near this point the trap appears to have sunk down, and 

 filled a hollow in the previously formed sandstone. This 

 mass of trap is at its greatest breadth 4^ feet wide, and is 

 almost of a circular form. At its upper extremity the 

 mass which connects it with the overlaying trap is 1^ 

 feet broad, and at its lower extremity there issues from 

 it a small vein 5 inches long and 2 broad. This ridge 

 continues to run southwards for about half a mile, and then 

 appears to unite with the southern extremity of the trap of 

 Salisbury Crags, and at its termination the porphyritic 

 greenstone affords a very beautiful example of the columnar 

 arrangement. The columns vary in length, thickness, num- 

 ber of their sides, and position ; those of the upper portion 

 of the mass being inclined at a great angle, while those of 

 the lower part are horizontal. Veins of compact and cry- 

 stallized prehnite very generally occur between the sides 

 of the columns, and in thickness these vary from half an 

 inch to 3 inches. 



The ridge of Salisbury Crags rises immediately above 

 St Leonard's Hill, from which it is separated by a deep val- 

 ley. Quarrying operations, and the making of the Radi- 

 cal Walk which runs along the base of the mural precipice 



