Septembeb 24, 1909] 



SCIENCE 



409 



Telopment of rare combinations are discussed. 



In Chapter V. Iddings gets at the meat of 

 the matter as to the formation of the rock 

 from the magma. The rock is formed by the 

 separation of solid, liquid and gaseous sub- 

 stances from the magma, crystallization of 

 minerals being the principal process. This 

 change from the magmatic solution to the 

 solid rock may take place in one or several 

 periods, at one or several levels in the crust of 

 the earth or at the surface. The conditions 

 under which different stages of consolidation 

 take place are constantly changing, not only 

 from the circumstances of environment inci- 

 dent to eruption but from the changes con- 

 nected with partial crystallization and various 

 disturbances of chemical equilibrium. It is 

 beyond the limits of this notice to review this 

 important chapter in detail and an enumera- 

 tion of the principal headings must suffice to 

 show the method of treatment. 



The separation of gases, liquids and solids is 

 discussed. Under the latter the principal 

 causes of separation are considered — such as 

 the addition or loss of substances, lowering of 

 temperature and changes of pressure. Satura- 

 tion and supersaturation as influenced by 

 changes in temperature, and the metastable 

 and labile conditions of solutions, are treated 

 in sections. Then comes the discussion of 

 Number of Points of Separation, The Rate of 

 Separation, The Effect of Viscosity on the 

 Rate of Separation, Polymorphic Substances, 

 The Order of Separation, Effect of Super- 

 saturation on Order of Separation, Separation 

 of Isomorphous Compounds, Zonal Structure 

 and Eutectic Mixtures of More Than Two 

 Compounds. 



The author refers to the subject-matter of 

 this chapter in these words : 



From the foregoing it appears that the solidifi- 

 cation with crystallization of rock magmas must 

 be an extremely intricate process, involving 

 variable or irregular changes in temperature and 

 pressure consequent on the movements of eruption, 

 together with variations in composition chiefly 

 through changes in gaseous components, and the 

 possibilities of chemical reaction among the com- 

 ponents with changing chemical equilibria, and 



the probabilities of supersaturation of the magma 

 by different components to various degrees. 



The origin of mineral composition of the 

 rock having been discussed, there is taken up, 

 in Chapter VI., the question of crystallization 

 and resultant texture. Here again the treat- 

 ment logically consists in showing the effect of 

 a great range of changing or variable condi- 

 tions in determining texture. After discus- 

 sing the genesis of the formal relations of the 

 parts of a rock Iddings describes the textures 

 of igneous rocks under the divisions Crystal- 

 linity. Granularity and Fabric, using the terms 

 which have recently been proposed' to supple- 

 ment the inadequate terminology in current 

 use. The illustrations used in this chapter are 

 particiilarly good and tend to emphasize the 

 desirability of greater precision and refine- 

 ment in the description of igneous rock tex- 

 tures. 



Differentiation of Rock Magmas is the 

 subject of Chapter VII. The evidences of 

 differentiation are first presented with regard 

 to the visible relations of parts of a single 

 rock mass on the one hand, and as exhibited 

 by many relations which connect the various 

 rocks of a petrographic province on the other. 

 Having established by the citation of facts 

 that the petrographer must recognize in vari- 

 ous rock series or groups the products of dif- 

 ferentiation from a parent magma, Iddings 

 proceeds to the discussion of processes and 

 hypotheses concerning them. It is notable 

 that this discussion is sane and conservative. 

 The author advocates no hypothesis or specu- 

 lation without a plausible basis in the laws of 

 physics and chemistry, which has sometimes 

 been done by prominent leaders in the science. 

 On the contrary, the conditions which have 

 been operative on the magmatic solution at 

 various stages of its history are examined to 

 find influences which may have led to differ- 

 entiation, of different degrees and kinds. The 

 effects of changes in density, viscosity, molecu- 

 lar concentration and saturation, are discussed, 

 to show that they may under certain circum- 



' " The Texture of Igneous Rocks," by Cross, 

 Iddings, Pirsson and Washington, Jour. Geol., 

 Vol. 14, 1906, pp. 692-707. 



