91 



getable soil. A few crustaceous lichen variola- 

 riae, leprarise, and urceolarise * were scattered 

 about upon the basalts. The lavas which are 

 not covered with volcanic ashes remain for 

 ages without any appearance of vegetation. On 

 the African soil excessive heat, and lengthened 

 drought, retard the growth of cryptogamous 

 plants. 



The basalts of Graciosa are not in columns, but 

 divided into strata 10 or 15 inches thick. These 

 strata are inclined under an angle of 80 degrees 

 to the north-west. The compact basalt alternates 

 with the strata of porous basalt and marl. The 

 rock does not contain hornblende, but great crys- 

 tals of foliated olivine, which have a triple cli- 

 vage This substance is decomposed with great 

 difficulty. Mr. Haiiy considers it as a variety of 

 the pyroxene. The porous basalt, which passes 

 into mandelstein, has oblong cavities from two to 

 eight lines diameter, lined with chalcedony, en- 

 closing fragments of compact basalt. I did not 

 remark that these cavities had the same direction, 



* We found the lecidea astrovirens, urceolaria ocelleta, u. 

 diamarta, (to which Mr. Achariur assimilates the lichen koe- 

 nigri of my Flora of Friberg) parmelia parietina, p. tenella, 

 (lichen hispidus Willd.) p. atra, lecidia fusco-atra, and many 

 other species, which were hitherto thought to belong exclu- 

 sively to the north of Europe. (Achar. Mcthodus JLichenum, 

 t . i, p. 152.) 



t Blaettriger olivin. 



