MINERALOGY. 309 



Feldspar. — The study of the chemical relationships of the feldspars has 

 deeply interested mineralogists for several years. It will be recalled 

 that Tschermak, by a laborious investigation, attempted to systematize 

 all those kinds of feldspar of intermediate content of silica by a theory 

 of isomorphism of the extreme members of the group. This theory 

 was prevented fi-om receiving general acceptation by the researches of 

 Des Cloizeaux, who gave definite optical properties for the oligoclase and 

 labradorite as well as the rest. But now Schuster, by a more extended 

 series of investigations, has been convinced that in optical as well as 

 other properties, all these intermediate species do form a regularly pro- 

 gressive series, such as might be expected to result from isomorphous 

 combmations. Many speculations have been made with the view of ex- 

 plaining how a lime alumina silicate like anorthite can be isomorphous 

 with potash or soda alumina silicates, which, like albite and microcline, 

 are of entirely different atomic ratio. The satisfactory understanding 

 of this is a thing to be determined when knowledge is further advanced. 

 The fact of the isomorphism is admitted, and the theory of the isomorphous 

 replacements to form intermediate feldspar species has gained ground. 



The dimorphism of potash feldspar has been known for some time. 

 But it has proved to be quite surprising to find that the new species 

 microcline is on the whole much more abundant than the old species 

 orthoclase, which previously embraced the whole. Microcline ditters 

 but extremely little in geometrical form from orthoclase, but its optical 

 properties are very markedly different. It is composed of a series of 

 laminse most intricately interwoven, a result of twinning according to 

 two difierent systems. Now Levy has lately shown that if this system 

 of twinning, which is sometimes carried to such an extent that the indi- 

 vidual lamince become minutely microscopic, were ouly repeated still 

 further, and the laminae subdivided until even the microscope could not 

 detect them, such a microcline would have all the properties, optical 

 and otherwise, of orthoclase. In this he is supported by able French 

 mineralogists, and it would appear as if we were liable to lose the old 

 classic orthoclase altogether. 



Other studies of twins.— A twinning plane becomes of necessity a plane 

 of symmetry, and thus by simple repetition of this process very regular 

 bodies result from the least symmetrical. The process of reducing well- 

 known minerals to less symmetrical systems has gone on as vigorously 

 as heretofore. For example, chabazite, which w^as considered as a most 

 typical rhombohedral mineral, has been shown by Becke to receive its 

 usual form by the repeated twinning of its triclinic crystals. Leucite, 

 which at the time of the last review had with general consent been re- 

 duced from the isometric to the quadratic system, has since been found 

 by Websky to be orthorhombic, and by others has been considered as 

 still less symmetrical. Perowskite and Boracite receive their apparent 

 isometric forms by the repeated twinning of rhombic crystals. Other 

 examples might be given, but these are sufticient to show what is being 



