176' Analysis of Scientific Books. 



enclosed rhomboidal ciystals of feldspar, of a deep flesh red, 

 from two to five lines long, as also prisms of augite or basaltic 

 hornblende. It contains, likewise, some thin plates of calca- 

 reous spar. The earthy and friable nature of this porphyry, 

 renders it susceptible of speedy decomposition when it is ex- 

 posed to the action of the weather. Hence one cannot look 

 without regret, on the crumbling substratum which now sustains 

 the lofty column reared on this hill to the memory of Lord Nel- 

 son. One of the principal effects of the action of the elements 

 has been to destroy and remove entirely, even to the depth of 

 several inches, the crystals of feldspar, and the little zeolites, 

 contained in the argillaceous paste, leaving in their place a mul- 

 titude of holes. The rock then resembles a porous lava. 

 Some mineralogists, deceived by this appearance, and observ- 

 ing but superficially the porphyry of the Calton-hill, have has- 

 tily concluded that it was the remains of a stream of lava, 

 poured out by a volcano. In the mass of the porphyry, we 

 find abundance of arsenical pyrites, and in the veins of calca- 

 reous spar which traverse it, we meet occasionally with a va- 

 riety of analcime, of a flesh-red colour, to which the name of 

 sarcite has been given. This mineral is opaque, its fracture is 

 unequal and earthy ; it is commonly found crystallized in a 

 solid of twenty-four sides, but the crystals are rarely single 

 and perfect. They occur rather in confused groups. 



To the south of the town we have two other hills of the trap 

 formation ; Salisbury-crags and Arthur's seat : the first under the 

 aspect of a steep mountain, terminated at the summit by a long 

 ridge of precipitous rocks, resembling the little Sal^ve ; the se- 

 cond loftier, and of a conical shape, recalls the figure of the 

 Mole as seen from Geneva. Mr. Necker classes the former 

 with the floetz trap of Werner, in opposition to Mr. Jameson, 

 who ranks it with the coal formation. It possesses, in fact, all 

 the geognostic characters of the trap formation. In this hill, the 

 scene of so many contests between the rival geologists of 

 Edinburgh, we have three objects worthy of successive exami- 

 nation. \st, The strata of sandstone, which compose its 

 base and most considerable part; 2c?, The mass of grun- 

 stein which forms its crest ; 2d, And, lastly, the phenomena 

 presented by the two genera of rocks at their points of mutual 

 contact. A distinct line of demarkation separates them, and 

 allows the circumstances which accompany this junction to be 

 clearly observed, over a long extent. The beds of sandstone 

 run from south-west to north-east ; they dip to the south-east 

 and rise to the north-west, forming, with the horizon, an angle 

 of about 60°. The mass of grunstein has no appearance of 

 stratification. It follows nearly the same direction and the 

 same inclination as the sandstone ; and the precipice is turned 

 towards the west, as are all the rocky facades near Edinburgh. 



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