CASTLE ROCK. 67 



bent up amongst the superincumbent trap. In its mineral 

 characters, the slate-clay exhibits marks of great alteration, 

 and, from being a soft slate, becomes changed into a rock 

 having little of a schistose structure, and so compact as to 

 afford an uneven conchoidal fracture. From this to the 

 Queensferry, there is no other junction of the trap and 

 sandstone visible ; to the west of the Queensferry, however, 

 there are, on the shore, one or two dykes of trap associated 

 with a tufa, the components of which are angular and on a 

 small scale. 



In the centre of the city of Edinburgh, the Castle Rock 

 rises to the height of 445 feet, forming an eminence which, 

 with the exception of its eastern side, presents on all quarters 

 a perfect mural ascent. The rock of the Castle is composed, 

 throughout its whole extent, of a basaltic clinkstone of a 

 greyish-black colour, which is in general arranged in more 

 or less perfect tabular concretions, and contains minute veins 

 of calcareous spar, sulphate of barytes, prehnite, and Wol- 

 lastonite. The forming of the New West Approach has dis- 

 played, in the clearest manner, the relations of the Plutonic 

 rock of the Castle to a series of sandstone and shale which 

 dips to the east at about 12°, and forms a gentle descent 

 to the Palace of Holyrood. In their position the strata ex- 

 hibit much disorder, being shifted and variously waved ; 

 and all, when in contact with the trap, become indurated, 

 the sandstone passing into a quartzose variety, and the shale 

 assuming a more compact structure. Near the gate of the 

 Castle, several masses of the sandstone may be observed 

 completely enveloped in the trap, all of which exhibit in- 

 tense induration. Concerning the more immediate relations 

 of the Castle Rock to the strata which occur on its western 

 side, little can be ascertained, from the almost entire absence 

 of openings, either natural or artificial ; it may be stated, 

 k2 



