HISTORY OF CRYSTALLINE ROCKS. 17 
will vary in different areas, and during different periods in the same area. It involves 
also a corresponding change in the nature of the materials dissolved, so that differences 
greater or less are to be looked for in the composition alike of eruptive plutonic and of 
crenitic rocks, when those of different areas and different ages are compared. The evi- 
dence of some such changes, even independent of aqueous action, in the composition of the 
plutonic mass, did not escape the acute observation of Durocher, and was in 1857 discussed 
by him in his remarkable essay on Comparative Petrology.' To this I called attention in 
1858, stating that in Durocher’s view the two strata of molten mineral matter imagined 
by him, “occasionally more or less modified by a partial crystallization and eliquation, or 
by refusion,” give rise to the principal varieties of acidic and basic crystalline rocks. * 
§ 25. This view was stated with great clearness by Durocher, who declared: “The 
magmas which have produced the igneous rocks are to be compared to metallic baths which, 
holding many metals in a state of fusion, separate in solidifying into different alloys, accord- 
ing to the circumstances of their solidification,’—these circumstances being “ conditions of 
an exterior rather than of an interior order.” Subsequently, in comparing a basic and 
highly aluminous phonolite with a trachytic porphyry, more siliceous and less aluminous, 
he remarks that an admixture of these in equal proportions would give the composition 
of a normal trachyte, and expresses the opinion that the rocks thus compared are probably 
“the two opposite products of an eliquation which took place in the midst of the liquid 
mass, as in the formation of two opposite alloys into which a metallic bath is so often 
seen to separate.’ These phenomena of eliquation he conceived to be very general in 
nature: “They must have taken place beneath the surface of the earth, and in its caverns 
and crevices, as well as at the surface.” 
§ 26. The probability of this view is apparent to all chemists who have studied the 
phenomena due to the crystallization and the different melting and solidifying points of 
metallic alloys, as, for example, the separation of lead from its silver-bearing alloy in the 
Pattinson process, and the eliquation of this metal from its alloy with copper. It was 
adopted by Macfarlane in 1867, in explanation of the relations of more or less basic horn- 
blendic and granitic rocks, already cited in § 21, and finds a striking illustration in the 
late experiments of Fouqué and Michel Lévy on the artificial production of crystalline 
mineral species from fused vitreous mixtures. From such a mixture, containing the ele- 
ments of six parts of chrysolite, two of pyroxene, and six of labradorite, kept at a heat 
near whiteness for forty-eight hours, there separated crystals of chrysolite, 0.5 millimetre 
in diameter, together with magnetite and spinel (picotite) ; a vitreous magma still remain- 
ing, from which crystallized, at a lower temperature, macled crystals of labradorite, with 
pyroxene, magnetite, and spinel, as before. It is apparent that with a greater lapse of 
time, and the formation of larger crystals of chrysolite, which has a specific gravity of 
about 3.4, these would, under the influence of gravity, subside, together with magnetite 
and spinel, from a fused glass holding the elements of pyroxene and feldspar, the more so 
as the density of fused doleritic and basaltic material is less than 2.8. From such a slowly 
cooling mixture the process of eliquation would, under favorable conditions, give rise to 
a highly chrysolitic aggregate, on the one hand, and to a dolerite with little or no chrys- 

1 Annales des Mines, xi. 217. A translation of this into English by Haughton was separately published in 
Dublin, in 1859, ? Chemical and Geological Essays, p. 3. 
: Sec. III., 1886. 3. 
