Reviews — Prof. Bosenbusch's Microphysiography. 123 



classification are those depending on the history of the science, and 

 the practical difficulty of referring the older rocks definitely to a 

 volcanic origin, owing to the " ephemeral" nature of volcanic forma- 

 tions and the easy removal of the tuffs by denuding agencies (pp. 6 

 and 346). These reasons will perhaps not appear conclusive to most 

 English geologists. 



At the outset the author divides igneous rocks, according to their 

 mode of occurrence, into plutouic (Tiefengesteine), volcanic {Erguss- 

 gesteine), and a group of intermediate nature occurring character- 

 istically in the form of dykes (Ganggesteine). The volcanic class 

 includes the tuffs as well as the lavas. 



The structure of igneous rocks is dealt with on the lines indicated 

 in the author's valuable paper in the Neues Jahrhuch (1882, vol. ii. 

 p. 1), in which he divides the rock-forming minerals into four groups, 

 and lays down the important law that in granular igneous rocks of 

 the normal type the minerals have consolidated in a definite order, 

 that of " decreasing basicity." The term granular is restricted to 

 those holocrystalline rocks in which each constituent mineral is 

 entirely of one generation, thus indicating a steady change of 

 chemical and physical conditions throughout the process of consoli- 

 dation. This is opposed to the porpliijritic structure, where some of 

 the minerals recur in a second genei'ation, giving evidence of two 

 distinct periods in the formation of the rock. The form of an indi- 

 vidual mineral in a rock may either be due to its own mode of 

 crystallization {idiomorphic), or dependent on causes other than the 

 molecular forces proper to the mineral {allotriomorphic) . The normal 

 plutonic rocks are characterized by a structure in which idiomorphic 

 constituents occur only in small proportion [JiijpidiomorpMc) : this is 

 contrasted with the panidiomorphic structure of a certain type of 

 dyke-rocks, in which nearly all the component minerals exhibit 

 more or less crystalline forms. 



The plutonic rocks are divided by the author, according to their 

 mineralogical constitution, into eight families. Here the chief new 

 features noticeable are the division of the felspars into alkali-felspars, 

 and lime-soda-felspars, superseding to some extent the old distinction 

 of orthoclase and plagioclase, and the introduction of a family of 

 plagioclase-nepheline-rocks (styled Theralites) to balance the elseo- 

 lite-syenites. The diabases are placed in the plutonic class, though 

 with an admission of their intermediate nature (p. 173). The head- 

 line " The Tuffs of the Diabase-rocks " under the class " Tiefenge- 

 steine " certainly has an anomalous appearance. 



As in the old edition, each family is fully treated under several 

 heads — mineralogical composition, classification, structure, and the 

 phenomena of metamorphism produced or suffered by the rocks. 

 There is much new matter, especially with reference to the local 

 variations [Facieshildungen) of rock-masses, the effects of contact and 

 mechanical metamorphism, and the secondary alterations of rock- 

 forming minerals. The bibliographical lists are much enlarged, and 

 the matter throughout is brought well up to date. 



The author next treats of the dyke-rocks, among which are found 



