derocher’s comparative petrology. 
163 
Changes of Composition in the Basic Layer .—Let us see if changes of 
the same kind have taken place in the composition of the lower ferro- 
calciferons layer of fluid, including the Diorites, &c. The Diorites are 
the most ancient of the basic rocks, and were ejected the most abundantly 
in the earlier geological periods. But towards the end of the secondary 
period, and during the tertiary, they have been generally replaced by 
the pyroxenic rocks, which present three principal types, viz., Mela- 
phyre, Basalt, and Dolerite. Their chemical composition is sensibly 
different, although they proceed from the same focus, and I have thought 
that to obtain the composition of the liquid layer from which they pro¬ 
ceed, the best method would be to take the mean of the compositions of 
the three types. We thus obtain a general term of comparison, which 
I have called in my Table, Pyroxenic Rock of average composition , and 
which represents the whole group of modern basic rocks as distinguished 
from the Diorites, which represent the ancient basic rocks. By comparing 
the numbers thus obtained with those presented by the Diorites, we can 
appreciate the changes which have taken place in the ferro-calciferous 
fluid layer, from the primary to the tertiary period. We see thus that 
there was a sensible diminution of silica and potash, and a notable aug¬ 
mentation of soda and lime. The proportion of soda has continued to 
increase still later, for our actual volcanic products contain still more than 
those of the tertiary period. The proportion of iron appears to have 
diminished rather than increased, but I should observe that the masses 
of magnetic oxide are connected with the Hornblende rocks, and it is to 
this circumstance that the richness in iron of the Diorites is due, while 
a special cause tends to impoverish some of the actual volcanic products; 
that is, the influence of Chlorine, which carries away the iron in the state 
of vapour. 
Similarity of the changes undergone by the Acid and Basic Layers .—- 
We may recognise a remarkable similarity in the changes experienced 
by the Acid and Basic layers; in both there has been a decided diminu¬ 
tion of Silica and Potash, while, on the contrary, the proportions of lime 
and soda have augmented. But the two layers remain, nevertheless, 
distinct, and the Trachytic products which represent the deeper portions 
of the Siliceous layer differ much less in the whole of their elements from 
the Granites (even the most ancient), than they do from the Diorites, or 
any other product of the basic layer. As to the hybrid rocks which issue 
from the zone of contact of the two layers, my Table shows that, as well 
by chemical composition as in mineralogical characters, they form a sort 
of tract of union between the two systems, although they seem to ap¬ 
proach somewhat nearer to the siliceous rocks. 
Causes of this Change of Composition. —The diminution of silica and 
potash in the modern rocks of the Acid and Basic group seems to me to 
arise from the fact, that those elements were concentrated towards the 
upper portion of the fluid zone, on account of their low specific gravity : 
and, on the contrary, the proportion of lime should increase with the 
depth. But the increase of soda in the eruptive products of the later 
