IxXVi PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



above named, I am inclined to think that a really metamorphic 

 origin may with mneh probahility be assigned to the quinary com- 

 pounds just mentioned for north-western Ireland and Norway, as 

 also for that of the Western Alps, whilst other varieties of granite 

 undoubtedly occur under geological conditions so dissimilar as to 

 require a different view of their formation. 



The various associations of the minerals which compose the igne- 

 ous rocks have received close attention in a work which has just 

 emanated from the pen of our laborious Poreign Correspondent Dr. 

 F. Senft*, in which the formation, the decompositions, and the che- 

 mical changes of mineral substances are enumerated with a fulness 

 of illustration which cannot but be highly useful to geological in- 

 quirers. His chapter on the family of the felspars is especially 

 welcome, as discussing the newer views which have been brought 

 forward upon a group of mineral species so important to the geologist. 

 Dr. Gustav Tschermak, in a valuable paper read to the Academy of 

 Sciences at Yienna, in 1864t, had proposed to simplify the subject by 

 considering that there exist only three distinct kinds of felspar, viz. 

 adularia, or the potash-, albite, or the soda-, and anorthite, or the 

 lime-felspar, whilst the others, which by previous authors have 

 been described as distinct species, are but mixtures of the above 

 kinds. Thus most of the opake orthoclase is, according to him, a 

 compound of adularia and albite, whilst oligoclase and labradorite 

 are, similarly, various mixtures of albite and anorthite. It is true 

 enough that the lamellar alternations of orthoclase and albite ob- 

 servable in the crystals from several localities, the coating of ortho- 

 clase with a rind of oligoclase in Finland, and the relation between 

 the composition and the crystalline forms of the several species 

 render a part of these views extremely probable. Dr. Eammelsberg, 

 in a recent review of the subject t, is satisfied that the best analyses 

 prove that the felspars containing lime and soda together are iso- 

 morphpus compounds of pure lime-felspar (anorthite) and pure soda- 

 felspar (albite), the isomorphism of which as a whole does not 

 depend on the number or the equivalence of the elementary atoms 

 which compose those species. He holds that this reasoning is far pre- 

 ferable to the view of such a mineral being a mixture of anorthite 

 and an analogously constituted soda-compound, a soda -anorthite, 

 and recognizes also mixtures of different species in the oblique or 

 monochnic felspar containing lime, iron, potash, and soda, or baryta. 

 The speculations which flow from such a view of the juxtaposition of 

 these mineral species, embracing the visible peculiarities of stracture 

 and the frequent curious changes which have modified the grouping 

 of their ingredients, will affect more deeply than we had expected 

 the reasoning on the genetic relations of the crystalline rocks. 



Many of the topics connected with the foregoing minerals, and 

 rock-structure at large, are examined in a somewhat novel point of 

 view by Dr. Vogelsang, in his recent work on the Philosophy of 



* Die krystallinischen Felsgemengtheile, von Dr. Ferdinand Senft. Berlin, 

 1868. t Poggendorff's Annalen. Bd. cxxvi. S. 39. 1865. 



I Zeitsclirift der deutschen geol. Gresellschaft, 1866. 



