50 M. Humboldt's Account of the 



of the pyramidal manganese-ore. Upon measurement, how- 

 ever, for which the small but beautifully formed and bright 

 crystals of this variety are better suited than any of the rest, 

 these also turned out to belong to a species different from the 

 pyramidal one formerly described. The angles which these 

 crystals afforded are given above as the dimensions of the spe- 

 cies. The results obtained from the remaining varieties are 

 not sufficiently consistent to be considered different from 

 these, and as, moreover, the colour of their streak and their 

 hardness coincide, we may safely consider them as belonging 

 to the same species. Some of the octahedral crystals, quoted 

 by Count Bournon,* for which he proposes the denomination 

 offer oxyduU manganisien, must also very likely be referred 

 to the brachytypous manganese-ore. He supposes their form 

 to be derived from the regular octahedron, but does not 

 quote any decisive proofs in favour of this opinion, which 

 is rendered necessary when a species nearly resembling it is 

 found to have, for its fundamental form, a four-sided pyramid 

 so little different from the regular octahedron. Those varie- 

 ties which have their solid angles replaced by four faces, may 

 perhaps belong to the pyramidal manganese-ore, as is men- 

 tioned in the observations annexed to that species, which was 

 likewise not distinguished as a species of its own at the period 

 of publication of Count Bournon's catalogue.-f- 



Art VIII. — Account of the Eruption of the Volcano of Jo- 

 rullo in Mexico. \ By Baron Alexander de Humboxxjt. 

 With a Section of the Mountain. 



T. o the east of the Pic de Tancitaro, the Volcan de Jorullo 

 (Xorullo, or Juruyo) was formed in the night of the 29th 

 September 1759. M. Bonpland and myself reached its cra- 



* Catalogue, p. 395. 



+ Dr Turner is occupied with the Analysis of the Species described in 

 this Paper, and will give the results in a subsequent Number. — En. 



X We have given the above abridged account of this remarkable vol- 

 cano, in reference to a new theory of its formation by Mr Scrope, which 

 forms the subject of the next Article. -See Humboldt's Essai Politique 

 sw la Nouvelle Espagne, his Essai Geognostique, p. 351, and his Rda~ 

 Hon Historique. — En. 





