REVIEWS 309 



throughout and forms a constant function of the chemical composi- 

 tion of the magma. Period of crystallization and temperature have 

 an important influence only on the structure of the resulting rock. 

 The second plate here reproduced (PL VII) shows the products of 

 crystallization from an anorthite-nepheline magma without magnesia, 

 consisting principally of corundum, anorthite, and nepheline. This 

 was fused in large masses, producing a well crystallized gray rock. In 

 the microscope the principal mineral is seen to be plagioclase prisms 

 in long rectangular form with distinct cleavage and multiple twinning. 

 Between the plagioclase laths is a groundmass consisting of nepheline, 

 magnetite and glass. The physical properties of this plagioclase are 

 essentially those of anorthite (An 6 Abj) with the following chemical 

 composition : 



Corundum is enclosed in the plagioclase in the form of circular plates. 

 The groundmass contains many small microlites of magnetite, form- 

 ing sometimes a rectangular network. Nepheline occurs in short, hex- 

 agonal prisms and irregular masses, forming the greater part of the 

 groundmass. There are, in addition, pleochroic yellowish corroded 

 crystalline flakes, which are probably lepidomelane. The glass base 

 occurs in variable quantity in different parts of the crucible. 



The systematic subdivision of aluminosilicate magmas, in relation 

 to these experiments, deserves especially thorough examination. The 

 greater part of known eruptives on the surface of the earth belongs to 

 the aluminosilicate group of magmas. The principal and most stable 

 components of magmas are silica and alumina, while the bases are vari- 

 able and easily replace each other to form both minerals and rocks. 

 Both Si0 2 and A1 2 3 are capable of crystallizing out in free form by 

 supersaturation, and both (according to the experiments of Thugutt 1 

 and the theoretical conception of Wernadskij 2 ) are capable of playing 

 chemically the part of acids. Silica and alumina are thus conceived to 

 have an analogous systematic significance in the classification of erup- 

 tive magmas, granite being a magma supersaturated with silica, and 



' St. Thugutt, Zur Chemie einiger Alumosilicate. N. J. f. M., 1895, B.-Bd. IX, 



2 \V. Wernadskij, Ueber die Sillimanitgruppe, sowie die Rolle der Thonerde in 

 Silicaten. Moscow, 1891 (Russian). 



