ARTHUR'S SEAT. 57 



neous rock, the sandstone acquires all the characters of gra- 

 nular quartz and horn stone, and frequently assumes a green 

 colour from containing a portion of the augite, which enters 

 into the composition of the greenstone. Resting upon the 

 greenstone is an alternating series of tufa and basalt. The 

 tufa varies in its composition; it is either formed of variously 

 sized fragments of previously existing rocks, as greenstone, 

 sandstone, shale, and limestone, or is a fine aggregate of red 

 ferruginous trap sand. In the tufa, a little to the south of 

 Sampson's Ribs, a mass of red sandstone is imbedded ; a 

 portion of which is visible for about eight feet, and has a 

 breadth of four. It is much changed, some portions acquir- 

 ing all the characters of jasper. Though the relations of this 

 tufa to the series of trap and sandstone, which we have just 

 mentioned, are not very evident, still it is highly probable 

 that it is of newer formation, and that it has been protruded 

 through them. 



The summit of the hill is entirely formed of a greyish- 

 black compact basalt, containing in some places large crystals 

 of basaltic hornblende. On examining that portion of the hill 

 which forms one side of the Hunter's Bog, this basalt is 

 found to traverse the tufa in the form of a great vein, and 

 to produce, at its planes of junction, changes similar to 

 those exhibited by aqueous formations, when in the same 

 situations. Near the summit of the hill, and from the western 

 side of the large vein, a smaller one takes its origin, and tra- 

 verses the tufa which it indurates. About half way down the 

 hill, the basalt divides into two distinct veins, being sepa- 

 rated from each other by the tufa. (Plate VII. Fig. 1.) 

 The basalt of these veins is arranged in irregular columns. 

 Owing to the thick covering of debris, the basalt of this 

 part of Arthur's Seat cannot be traced to the base of the 

 hill ; it is, however, probable that if it could be so examin- 

 ed, it would be found to rise directly through it, and also 



