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THE GEOLOGIST. 



M. Descloizeaux, a distinguished mineralogist of Paris, who won 

 himself some reputation by his geological mission to Iceland, has been 

 studying for a long time the optical properties of crystals. He has 

 lately addi^essed to the Academy of Sciences a new memoir on this 

 subject, and hopes soon to have completed his numerous observations 

 on double refraction, principal axes of refraction, their number, posi- 

 tion, and relations to the optical axes, the laws of dispersion, &c., in 

 transparent minerals. It appears that the number of transparent 

 substances in the mineral world, including those which are transparent 

 enough when taken in thin laminae to give passage to a ray of light, 

 is about 180. Of these, 166, of which eighty-one have one axis, and 

 eighty-five two axes, have been completely studied by M. Descloizeaux. 

 Twelve alone remain of which the optical co-efficients are not yet satis- 

 factorily determined. 



We have alluded in former papers to the action of metamorphism 

 by eruptive rocks on combustibles (lignite, coal, &c.), also to the 

 action of lava and trap-rocks on limestones, argillaceous strata, and 

 sandstones. We have now before us a new memoir by M. Delesse, in 

 which granite is the eruptive rock under consideration. 



When metamorphism is studied with respect to granite-rocks it is 

 seen that their effects differ notably from those produced by the 

 different varieties of trap. The following are the characters presented 

 by strata that have undergone metamorphism by contact with 

 granite. If the rock acted upon be limestone, it often happens that it 

 has not been modified at all, even where it has been penetrated, or 

 even where it has been covered over, by granite. The giauconite, so 

 frequent in calcareous strata, remains also unaltered. More frequently, 

 however, the structure of the calcareous rock has become crystalline, 

 and of a paler colour, having passed into saccharoid limestone. If the 

 limestone be argillaceous, it has become very compact and lithoid, but 

 not silicified. In some cases it has become cavernous, but has not 

 passed into dolomite ; oftentimes, indeed, it contains less magnesia 

 where it is in immediate contact with the upheaved granite. Among 

 the minerals that have been developed in limestone under these cir- 

 cumstances, we must name more especially spathic carbonates, quartz, 

 and minerals common in metallic lodes. The latter form serpentine 

 veins in the metamorphosed rock, or line some of its cavities. 



When a siliceous rock has been upheaved by granite, we observe 

 that the metamorphism has been equally irregular ; sometimes com- 

 pletely null j sometimes so complete that the whole rock has been 

 transformed into transparent quartz. Quartz must in this case be 

 noted as the most important mineral developed in immediate contact 

 with granite. Next comes sulphate of baryta, with which the quartz 

 is often associated, fluor-spar, and the minerals of metallic lodes. 



When the rock modified by gTanite is argillaceous, its structure has 

 become schistose or lithoid. In some cases this structure approaches 

 to that of jasper ; but it has not been observed to have taken a 

 vitreous aspect (as if quartz had been formed). When the argillaceous 



