NOME III. BASALTIN WITH PORPHYRY. \QQ 



but as the ancient authors are unanimous that 

 the basalt came from Abyssinia, it probably oc- 

 curred under the appearance of columns, of too 

 small a diameter to be employed in architecture 

 or monuments. It is to be regretted that the 

 mountains of Abyssinia have not been explored 

 by any geologist, as the transcendent beauty of 

 the emerald-green granite alone might invite a 

 research into that interesting region. 



NOME III. BASALTIN WITH PORPHYRY. 



Basaltin being the base of porphyry, it is na- 

 tural to expect many examples of this kind. 

 Among others, near the village of Renaison, in 

 the department of the Loire, there occur, after 

 passing through fragments of granite, rocks of 

 black trap, surmounted by porphyry of the same 

 base, the transilience being clear and palpable. 

 This porphyry is crowned by another porphyry, 

 of a brownish grey ; but in this the crystals of 

 felspar are long, and thinly scattered (a por- 

 phyron); while the others are white, and fre- 

 quent. The black porphyry, and even the grey, 

 are harder than the trap. 



The separation of the trap or basaltin from 

 the porphyry is clearly marked by an undulat- 



