552 Geological Society : — '^-^ ^^ 



The author finds that the black-coloured Cryolite, when heated to 

 a dull redness, loses about 1 per cent, of moisture and acid, and 

 becomes less translucent ; and he considers that the Cryolite was 

 originally dark-coloured, and that the white and somewhat less 

 compact and more opaque variety has resulted from the action of 

 some external cause ; possibly, he thinks, in this case the agency of 

 overljnng igneous rocks. Two vertical trap-dykes traverse the cliflf 

 near by, one on either side of the Cryolite ; and, although no such 

 rocks now overlie the spot in question, the author thinks it quite 

 probable that the known powerful effects of the atmospheric agencies 

 in the Greenland climate may have removed all traces of the over- 

 lying eruptive rocks, to the former existence and influence of which 

 he refers the change of condition seen in the superficial Cryolite. 



2. " Description of remarkable Mineral Veins." By Professor 

 D. T. Ansted, F.R.S., F.G.S. 



This communication was introductory to a series of notices of 

 remarkable mineral veins, which the author has in contemplation, 

 and comprised a notice of the Cobre Lode of Santiago de Cuba. 

 The author commenced his memoir by a brief statement of the sense 

 in which he used the term "mineral vein," together with some 

 remarks on the nature of mineral veins, and the enumeration of 

 various kinds of observations needed in preparing such a report on 

 mineral veins as shall be useful for reference in subsequent investi- 

 gations for scientific purposes. 



The great Cobre lode, which Prof. Ansted selected for his first 

 notice, as being a very exceptional and remarkable vein, has been 

 known for twenty years as the richest copper lode worked for a con- 

 tinuance during that period. It is remarkable for its great magni- 

 tude and complication, its extraordinary richness, the high degree of 

 mineralization of the surrounding " country " (or enclosing rock), 

 and the nature of the adjacent rock-masses. The lode is opened in 

 a hill (about 600 feet above the sea), near the tnwn of El Cobre, 

 about eight miles W.N.W. of Santiago de Cuba. It occurs in a large- 

 grained calcareous porphyritic rock, which towards the south forms 

 a mountain- chain, and is associated with basalts and conglomerates. 

 To the north are hard limestones. The general direction of the 

 mountain-range and watershed, and the strike of the porphyries and 

 conglomerates, and of the lode itself, are nearly east and west. The 

 dip of the lode and of the bedded rocks is to the south, but the former 

 is more vertical than the latter. To the east, and probably also to the 

 west, the coast-range is syenitic. The south-eastern portion of Cuba 

 is subject to frequent earthquakes ; the central and western portions 

 are free from them. 



The outcrop of the Cobre lode, as at present known, extends 

 about a mile, but probably ranges further to the east. Near its 

 eastern known extremity a branch is given ofi^ at an angle of about 

 30°, which goes to the south-west. This branch is, in the author's 

 opinion, continuous with the Santiago copper-lodes, cropping out to 

 the south of the Cobre, which, though yielding some fine ore, have 

 not as yet at all equalled the Cobre lode in productiveness. 



