dtjkocher’s comparative petrology. 
161 
the second belong the Diorites, Ophites, Euphotides, Hyperites, Mela- 
phyres, Traps, Basalts, and Pyroxenic Lavas. 
Origin of Mineralogical Differences in Igneous Docks _I should also 
remark, that if we analyze various kinds of the same group of rocks—■ 
granites, for example—we often find more difference in the relative pro¬ 
portions of elements, between two specimens of the same type of rock, 
than there is between granite and a rock altogether dissimilar in ap¬ 
pearance,—say a Trachyte or Pumice. We should conclude from this 
fact, that in rocks derived from the same Magma differences of minera¬ 
logical composition arise less from their elementary composition than 
from conditions of pressure, temperature, and in general the circum¬ 
stances of their cooling; that is to say, from conditions of an external 
rather than an internal order. The Magmas which have produced the 
igneous rocks are comparable to baths containing many metals in a state 
of fusion, and which, in setting, are divided into alloys, different, accord¬ 
ing to the circumstances of their cooling, even when the original bath 
has the same composition. 
Products of the Zone of Contact of the two Magmas .—Moreover, the 
zone of contact of the two Magmas should give out products of an inter¬ 
mediate character; and this is, in fact, the case; and from this zone 
appear to arise the Syenites, the Protogenes rich in talc, the Trachytes 
rich in pyroxene and amphibole, and various porphyries which are in¬ 
termediate between Granitic or Trachytic porphyries, and Amphibolic 
or Pyroxenic porphyries. These rocks, which may be called hybrid rocks , 
have petrographical and geological affinities of an unsettled character; 
they seem to belong, sometimes to the rocks of the first, and sometimes 
to those of the second Magma. 
Permanence of Separation of the two Magmas. —The TJpper Magma, 
which is rich in Silica, and poor in earthy bases and oxides of iron, 
possesses the least specific gravity; and in this respect there are diffe¬ 
rences among the rocks produced by the two Magmas, from one and a 
half to twice as great as between oil and water. The separation is still, 
greater if, in place of considering the rocks in their natural condition, 
we compare the vitrified products obtained by their fusion : further still 
if we refer them to their liquid condition, there ought to be, according 
to Bischoff’s experiments, between the rocks arising from the two Mag¬ 
mas, differences twice greater than those observed in their crystalline 
state, and, therefore, from three to four times greater than those between 
oil and water: from these facts may be deduced the necessary and per¬ 
manent separation of the two Magmas. 
Fluid Zone situated below the Solid Exterior Crust. —The solid crust 
of the globe, then, reposes upon a fluid zone, composed of two distinct 
layers: the upper, which is the most refractory, is only semi-liquid or 
pasty, in consequence of the predominance of Silica, which is charac¬ 
terized by its viscosity; the second layer, which contains much less silica, 
and which presents atomic proportions, ranging from a bisilicate to a 
sesquisilicate, is much more fluid and dense ; and also appears to be very 
