Lewinson-Lessing s Classification of Rocks. 363 
(2) The effect of specific gravity. 
The liquid magma differentiates according to specific grav- 
ity. The heavier parts, which are basic, fall to the bottom; the 
lighter (Na^O, SiO--, etc.) float. Further, the first crystals 
formed float or fall according to their specific gravity; for ex- 
ample, in the magma of Vesuvius augite falls to the bottom and 
leucite floats and remains in the superficial parts of the lava. 
Spheruliths are less heavy than the rock. That is whv they 
are found upon the borders forming randliche Ausbildungs- 
form (Fuchs). Hence the specific gravity influences the differ- 
entiation of the magma. According to the researches of F.Kiis- 
ter, in a fused mixture of isomorphous bodies the heaviest are 
am.ongst the first to be deposited. The author supposes this 
rule is applicable to magmas. 
(3) Tlie role of pressure. 
It consists in the following: 
(a) Pressure can raise or lower the temperature of fusion. 
(b) It retains in the magma the mineralizing agents, steam, 
etc. 
(c) It prevents the dissociation of combinations which de- 
compose at a high temperature. 
(d) It facilitates or retards the liquation of magmas that 
are immiscible. 
(e) It can separate a part of a magma by compression. 
(f) It may have some influence on the structure of the 
minerals formed. 
(a) The first point has been treated. 
(b) From this point of view submarine crystallization is 
very interesting. De Stefani has shown that the pressure of 
the water of the ocean does not avail to retain in lava the min- 
eralizing agents, that is, the gases; and as lava cools quicker in 
water, there ought to result glasses and porous lavas, as con- 
firmed by observation. The author thinks that when once the 
exterior amorphous crust has been formed the magma is pro- 
tected by this crust, cools slowly and crystallizes. He thinks 
that the diabases are formed in this manner. These rocks are 
later denuded, and the primitive crust is rarely encountered. 
Yet Rosenbusch affirms that diabases are met with "mit ge- 
flossener Oberflasche." 
