60 GEOLOGY OF THE LOTHIANS. 



ture of this greenstone in some places approaches to a glo- 

 bular mode of arrangement, which is rendered very conspi- 

 cuous by red haematite, forming a minute layer between the 

 concretions ; it is traversed also by a vein of compact green- 

 stone about six feet in breadth. Porphyry forms the prin- 

 cipal mass of the hill, having a base in general of a bluish- 

 grey claystone, while in others it assumes the characters of 

 an earthy greenstone, containing crystals of basaltic horn- 

 blende and flesh-red felspar. In many places the porphy- 

 ritic structure is associated with the amygdaloidal, thus 

 forming an amygdaloidal porphyry ; the cavities in general 

 contain only calcareous spar ; but flesh-red cubicite has been 

 found in some places in the same position. Trap-tufa oc- 

 curs in the Calton Hill associated with the porphyry, and 

 at the back of the new High School it forms considerable 

 beds ; the components of this tufa are in general in a state 

 of minute division, and are principally composed of an in- 

 durated wacke. In regard to the relations of the ignige- 

 nous mass of the Calton Hill to the stratified rocks, it may 

 be stated that it appears to have been protruded through 

 them, in a direction more or less parallel to that of the strata. 

 During the excavating operations which were carried on 

 for the foundation of the Waterloo Bridge, sandstone strata 

 were found underlying the trap of the hill, and dipping to 

 the east at a considerable angle. Resting upon the trap, 

 and also dipping east at angles, which decrease as the dis- 

 tance from the upraising agent increases, is that series of 

 sandstone, conglomerate, and shale strata, which commences 

 at the west end of Regent Terrace and terminates at the 

 sea. At the stairs which lead up from the Waterloo Bridge 

 to the hill, several strata of a red trap-tufa are found com- 

 pletely imbedded in the porphyry ; they dip like the sand- 

 stone upon which the Regent Terrace is built, to the east 



