January 3, 1902.] 



SCIENCE. 



33 



is offered. Briefly stated, the essential idea is 

 that, just as in a highly cooled vessel of salt 

 water the ice crystallizes at the sides, bottom 

 and top, leaving a core of more concentrated 

 liquid at the center, so here the solvent may 

 have frozen out, collecting at the borders of 

 the cavity in a more or less pure condition, as 

 foyaite, and gradually becoming more basic 

 (richer in the solute) as the freezing process 

 crept towards the center. 



Although the great work of the Russian 

 petrographer, F. Loewinson-Lessing, on the 

 Eruptive Rocks of the Central Caucausus, was 

 issued more than two years ago, the views ad- 

 vanced are only beginning to get into form 

 accessible to the majority of English students. 

 The general interest lies in the discussions of 

 the subjects of rock-classification and the dif- 

 ferentiation of rock magmas. 



The classification proposed for the igneous 

 rocks is chemical. It is based primarily upon 

 the degree of acidity of the silicate minerals. 

 Four great groups are thus established: (1) 

 The ultra-basic rocks, derived from a mono- 

 silicate magma, (2) basic rocks, which had a 

 bisilicate magma, (3) neutral rocks, with a 

 magma which was' bisilicate or normal, and 

 (4) acid rocks, in which the magma was poly- 

 silicate. These groups are subdivided in 14 

 sub-groups and 34 families. 



In order to find the proper systematic posi- 

 tion of an eruptive rock from the fundamental 

 viewpoint of the proposed classification four 

 factors are considered: (1) The relation of the 

 oxygen in the silica and that in all the other 

 oxides taken together, giving what is termed 

 the coefficient of acidity; (2) the chemical 

 composition, which gives for each type a dis- 

 tinctive formula; (3) the relations between 

 the two groups of oxides according to their 

 molecular proportions; and (4) the relations 

 of the soda and potash in the alkaline rocks. 

 This consideration of the principles of classi- 

 fication leads to the proof of the distinct 

 phases of fundamental magmas. 



Discussion of the differentiation of rock 

 magmas has an unusual interest. The Russian 

 author calls special attention to the principle 

 of Soret, the action of super-saturated solu- 

 tions, the effect of gravity, the principles of 



maximum work as proposed by Berthelot, and 

 the reaction of mixed liquids, as operating in 

 the separation of magmas. 



Three distinct kinds of magmatic differen- 

 tiation are recognized. They are : Static dif- 

 ferentiation, taking place in the depths of the 

 earth; differentiation by cooling during ascent 

 to the surface; and crystalline differentiation. 

 Specific gravity, pressure and temperature are 

 the chief factors governing the course of the 

 static kind ; while chemical affinities come into 

 play in large measure only in crystalline sepa- 

 ration. 



The role of inclusions of foreign rocks, 

 which has so long been such an unsatisfactory 

 subject to petrographers, is explained on the 

 idea that it is only that portion of the magma 

 yet undifferentiated which affects the intro- 

 duced rocks. After thorough assimilation of 

 limestone, for example, a separation of the 

 modified magma takes place. One part con- 

 tains very little lime and the other nearly all 

 of it. Rock formed from the first mentioned 

 might be a granite, while from the second 

 would come perhaps -a gabbro. 



Charles R. Keyes. 



on the reason for the retention op salts 

 near the surface of soils. 



Very recently a light-colored saline incrus- 

 tation was noticed by Professor Milton Whit- 

 ney upon the surface of the soil in the grounds 

 of the Department of Agriculture in Washing- 

 ton. This crust was collected and examined in 

 the laboratory of the Bureau of the Soils under 

 the direction of Dr. Frank K. Cameron. The 

 crust contained about 1 per cent, of soluble 

 matter, principally sulphates and nitrates of 

 sodium and calcium. Samples were then col- 

 lected at different depths and examined to de- 

 termine the vertical distribution of the soluble 

 salts. The results showed that although the 

 soil was examined to a depth of three feet, 

 practically all of the salt was in the surface 

 inch, the larger part of it being in the top 

 eighth-inch. 



The crust was found at the end of a short, 

 dry season, such as is common in the autumn 

 months along the Atlantic coast region. 



A number of similar occurrences of ab- 

 normal amounts of soluble matter on the sur- 



