Review of Recent Geological Literature. i8i 
Les analyses en bloc et leur interpretation, par M. F. Fouque. (Bull. 
Soc. Fr. Mineral, 1902, XXV., pp. 278-359). 
In attempting a review of this important work it has seemed that 
a few quotations will best serve the purpose. 
"Block-analyses of eruptive rocks have found great favor of late 
years among petrographers." * * * 
"When a rock is holocrystalline, and when one has made separately 
and beforehand an analysis of each one of the constituent minerals it 
is possible to deduce from the block-analysis the quantitative mineralog- 
ical composition, a fact, evidently, of some importance. But if the con- 
dition named is not fulfilled it must be admitted that only controvertible 
results are obtained even when the microscope has revealed ♦the nature 
of the constituents of the mineral mixture studied." * * * 
"There are few holocrystalline rocks which do not belong to the 
category of rocks called plutonic, which are characterized for the most 
part by the presence of crystals that are rarely encountered among the 
mineral substances derived from the action of volcanoes of the present 
time. Volcanic rocks are formed essentially by means of igneous fusion. 
The minerals of the plutonic rocks are intimately related, on the con- 
trary, to products crystallized in veins by the aqueous method." * * * 
"In plutonic rocks the frequence and sometimes even the pronounced 
predominance of quartz, muscovite, tourmaline, fluorite, etc., exclude 
the idea of a production directly from a molten magna and show that 
nature has here availed herself chiefly and sometimes solely of a modus 
operandi entirely dififerent. If the rocks in question emanate primordi- 
ally from a fused silicate magma in the midst of which their elements 
have temporarily existed under the influence of pressure, they are now 
only indirectly connected with this magma ; they have been, as it were, 
extracted from it and transported a greater or less distance by the ac- 
tion of mineralisers ; they have thus lost the imprint of their first home, 
and acquired characters essentially different from those possessed by 
their original matrix. They are as far removed from it as the prod- 
ucts of volatilisation, of decantatiou or of crystallisation from an aque- 
ous solution can be from their primitive host." * * * Therefore, 
chemical analyses of these rocks "have no value so far as their origin 
is concerned." 
These considerations "do not apply to the study of rocks called vol- 
canic. These form before our eyes in nature, we see them derived di- 
rectly from igneous fusion, and we can easily reproduce in our labor- 
atories most of them by fusion of their chemical elements and properly 
controlled resolidification. The connection between the solid rock and 
the fused magma from which it is derived can be followed step by step. 
Modifications produced by crystallization hardly change the composition 
at all." 
The author then proceeds to a detailed study of a large number of 
mass analyses of the volcanic rocks of Santorin with especial reference 
to the compositiqn of the feldspar as derived from the composition of 
the rock. He shows that there are two chief varieties of volcanic rocks 
