Jan. 1832. 



GEOLOGY. 



5 



This unusual degree of atmospheric dryness was accompanied 

 by continual flashes of lightning. Is it not an uncommon 

 case^ thus to find a remarkable degree of aerial transparency 

 with such a state of weather ? 



The geology of this island is the most interesting part of 

 its natural history. On entering the harbour^ a perfectly 

 horizontal white band^ in the face of the sea clifl^, may be seen 

 running for some miles along the coast^ and at the height of 

 about forty-five feet above the water. Upon examination^ 

 this white stratum is found to consist of calcareous matter^ 

 with numerous shells embedded^ such as now exist on the 

 neighbouring coast. It rests on ancient volcanic rocks^ and has 

 been covered by a stream of basalt^ which must have entered 

 the sea^ when the white shelly bed was lying at the bottom. 

 It is interesting to trace the changes^ produced by the heat of 

 the overlying lava, on the friable mass. For a thickness of 

 several inches it is converted^ in some parts^ into a firm 

 stonC;, as hard as the best freestone ; and the earthy matter^ 

 originally mingled with the calcareous^ has been separated into 

 little spots_, thus leaving the limicstone white and pure. In 

 other parts a highly crystalline marble has been formed, and 

 so perfect are the crystals of carbonate of lime, that they can 

 easily be measured by the reflecting goniometer. The change 

 is even more extraordinary, where the lime has been caught 

 up by the scoriaceous fragments of the lower surface of the 

 stream ; for it is there converted into groups of beautifully 

 radiated fibres resembling arragonite. The beds of lava rise 

 in successive gently-sloping plains, towards the interior, 

 whence the deluges of melted stone originally proceeded. 

 Within historical times, no signs of volcanic activity have, I 

 believe, been manifested in any part of St. Jago. This state 

 of quiescence is, probably, owing to the neighbouring island 

 of Togo being frequently in eruption. Even the form of a 

 crater can but rarely be discovered on the summits of any of 

 the red cindery hills ; yet the more recent streams can be dis- 

 tinguished on the coast, forming a line of clifl's of less height, 

 but stretching out in advance of those belonging to an older 



