MINERALOGY. 663 



description of the microscopic constitution of meteorites, and will consist 

 largely of photographic illustrations, together with such descriptions 

 and explanations in the text as the subject calls for. The preparation of 

 the illustrated plates has been intrusted to J. Grimm, of Offenburg, 

 who has carried through in so admirable a manner the work of Cohen 

 devoted to the illustration of the microscopic structure of minerals. 

 One part of this work on meteorites has appeared, and the remainder 

 will be looked for with iuterest. It promises to supply a long-felt need, 

 that of a general work which shall present, in systematic form, for 

 the benefit of the many interested in the subject, the large amount of in- 

 formation which the microscopic study of meteorites has led to. 



CRYSTALLOGRAPHY AND PHYSICAL MINERALOGY. 



A very large uumber of articles have been published devoted to a de- 

 scription of the crystalline form of minerals, but most of them are of 

 too limited iuterest to require notice here. Attention will be called only 

 to a few of more general character. The species of the feldspar group 

 have been the objects of much study both with reference to their crys- 

 talline form and their optical properties. Klockmaun, for example 

 (Zeitsch. Kryst., vi, 493), has studied the various kinds of twin-crystals 

 of orthoclase which occur in the granitite of the Eiesengebirge, with the 

 result of adding materially to our knowledge of this already complex 

 subject. Forstuer (Zeitsch. Kryst., vin, 125, also, I, 547) has devoted 

 himself to the feldspars which occur in the volcanic rocks of the island 

 Pantelleria. He has proved, in the first place, the existence there of a 

 sorfa-orthoclase, monocliuic in crystallization and near albite in angle. 

 The composition corresponds to one molecule of orthoclase, or potassium 

 aluminum silicate, and two molecules of an analogous sodium-aluminum 

 silicate. Other eases of a sodium-bearing orthoclase have long been 

 known, but the sodium plays here a more prominent part, and the 

 crystallographic and optical data given by Forstuer are of great value 

 in showing the relation of this monocliuic member of the feldspar 

 family to the potassium-bearing varieties of the triclinic feldspars or 

 plagioclase. Forstuer's investigation of the plagioclase feldspars of the 

 same locality is most complete, and yields many interesting points which 

 can be only hinted at here. The interest connected with this group of 

 feldspars has been much increased since the general acceptance of 

 Tschermak's well-understood theory as to their relation in composition, 

 and since the optical investigations of Schuster have shown that in 

 optical characters there is a transition analogous to that in composition 

 from the one extreme (anorthite) to the other (albite). The feldspars 

 examined by Forstuer form a series with little calcium and rich in so- 

 dium, and hence approximating to albite; but at the same time the un- 

 usual large percentage amount of potassium separates fhem from the 

 ordinary plagioclase. The author shows, however, that they can be con- 

 sidered, in the sense of the Tschermak theory, as isomorphous mixtures 



