124 GEOLOGICAL & MINERALOGICAL STRUCTURE OF 



of the hill are composed. At the junction of these rocks, the passage is 

 sometimes so gradual as to give the intermediate rock an indeterminate 

 character partaking of the nature of both. At others, it is abrupt, yet 

 notwithstanding the abruptness of the change, the vertical and horizontal 

 fissures are prolonged into each and cross the line of junction, I shall 

 not here enter into a greater detail of these appearances, but shall con- 

 tent myself with observing that the most satisfactory explanation of 

 these phenomena, is derived from that theory which ascribes to the trap 

 rocks an igneous origin, under pressure of a great body of water. 



The semi-columnar basalt forming the greater part of the hill is very 

 porous, containing numerous amygdaloidal cavities, which are for the 

 most part merely lined with a peculiar mineral, which I presume to name 

 ^conchoidal aiigite ; sometimes, howe^^er, they are nearly filled with it, or 

 with calcedony, semi-opal, or carbonate of lime ; the calcedony being 

 usually covered with a coating of green earth. The rock itself is com- 

 posed of hornblende and felspar, with the augite, so profusely disseminat- 

 ed, as to claim a right to be considered in some cases, as a constituent of 

 the rock. This is, I believe, the first time that conchoidal augite has been 

 found entering into the composition of basalt. The rock is fusiblet and 

 is of considerable specific gravity, notwithstanding its porosity. The 

 vertical and horizontal fissures are not always straight, but are at times 

 waved ; they are also, sometimes lined with an infiltration of calcedony 

 coated with clay and chlorite. 



This 



* From the difficulty of procuring specimens sufficiently large to analyse, I have not been able 

 to determine its composition exactly. 



f \ 1 have lately had an opportunity effusing a large piece of the Sitdbald'i basalt in a steel 

 furnace ; the product after an hour's fusion was a fine black opake stone, resembling porous 

 obsidian : glass bottles are commonly made in the Mediterranean from basalt, and that of SUU' 

 bald', seems to be of an equally favourable nature for that purpose. 



