MPeRSDAY,~ PULY 25 
QRIGIN OF IGNEOUS ROCKS. 
Igneous Rocks and their Origin. By Prof. R. A. 
Daly. Pp. xxii+563. (London: Hill Pub- 
lishing Co., Ltd.; New York: McGraw-Hill 
Book Go., Inc., 1914.) Price .17s.net. 
ROF. DALY is a man of ideas. A few facts 
observed in the field suggest to him a 
hypothesis which he then proceeds to test by 
searching for other facts which must exist if the 
hypothesis be of any value for scientific purposes. 
He is perhaps most widely known as the author 
who, more than any other, has developed the 
theory of ““magmatic stoping.” Large masses of 
plutonic rock—such, for example, as the granites 
of Devon and Cornwall—can be proved by field 
evidence to fill spaces that must formerly have 
been occupied by other rocks. In the case re- 
ferred to the displaced rocks consisted largely of 
folded sediments. What has become of them? 
According to the theory in question, the roof of 
the magmatic chamber has been shattered, and 
the detached fragments have in general sunk in 
the rising plutonic magma. This theory is ex- 
plained and illustrated in the present volume, and 
much use is made of one of its probable conse- 
quences—namely, the development of secondary 
magmas by “syntexis’’; or, in other words, by 
the solution in the rising magma of the masses 
detached from the walls and roof of the magma 
chamber. This action is believed to account 
directly or indirectly for many varieties of igneous 
rock, 
But the theory of magmatic stoping is only a 
subsidiary feature of the work. Its main object 
is to explain the known facts of igneous geology 
by a few general assumptions as to the composi- 
tion, structure, and physical condition of the 
planet, and whatever view is taken as to the 
validity of the assumptions, there can be no doubt 
that in working out their consequences the author 
has produced a most interesting work, full of 
information on the present-day aspects of igneous 
geology, and eminently calculated to stimulate 
thought. 
the book is divided into three parts. The first 
deals with the composition and mode of occur- 
rence of igneous rocks, the relative abundance of 
the different types at the surface of the earth, and 
the phenomena of active volcanoes. Rosenbusch’s 
classification is adopted with slight modifications. 
The classification based on the “norm” is dis- 
carded as being useless for the object which the 
author has in view. The second part deals with 
abyssal injection, magmatic stoping, assimilation, 
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449 
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erentilationy- the mechanism of volcanic 
vents, and concludes with a statement of what 
the author terms an eclectic theory of igneous 
rocks. By the term eclectic he means to imply 
that in framing the theory he has selected and 
appropriated whatever seemed to him best in the 
earlier theories relating to the same subject. The 
third part is devoted to applying the general 
theory to igneous rocks, which, for this purpose, 
are divided into seven great groups or clans: the 
gabbro-clan, the granite-clan, the diorite-clan, the 
granodiorite-clan, the syenite-clan, the alkaline- 
clans, and the peridotite-clan (including magmatic 
ores). 
The eclectic theory may be briefly summarised 
as follows :—The earth, regarded as a planet, is 
roughly stratified according to density, but the 
three outer shells are alone involved in the pro- 
duction of the igneous phenomena with which 
geologists have to deal. The outer shell is com- 
posed of sediments, with an admixture of volcanic 
material, and is discontinuous. The second shell, 
represented by the Canadian and Fennoscandian 
“shields,” approximates to granite in composition. 
It is probably continuous under continental areas, 
but may not be present under all the oceanic areas, 
These two shells collectively form the “crust” of 
the earth. Beneath them is a third shell or sub- 
stratum of basaltic composition which alone, 
“since an early pre-Cambrian period (typified in the 
Keewatin) has been not enough for spontaneous 
eruption.” It may be discontinuous, but, if so, 
parts of it underlie both oceanic and continental 
areas. Abyssal injection implies the rise of the 
material of the substratum in magmatic wedges 
which are superheated at the higher levels and 
therefore capable of dissolving the rocks of the 
crust to a variable but large extent. Both the 
primary basaltic magma and each of its solutions 
with crust-rocks are subject in certain conditions 
to magmatic differentiation, this giving rise to 
various magmas by the freezing of which the dif- 
ferent types of igneous rock have been produced. 
A few illustrations of the way in which the author 
applies the theory will now be given. 
_ The composition of the primary basaltic magma 
is regarded as that of a basalt containing only a 
moderate amount of olivine. From such a magma 
basalts and gabbros rich and poor in olivine may 
be derived by gravitative differentiation to which 
the author attaches great importance. Peridotites 
and anorthosites may be regarded as the extreme 
phases of the differentiation of the primary 
magma. Quartz-basalts and related rocks which 
are now known to be widely distributed in con- 
tinental areas, though apparently absent from 
oceanic areas, probably owe their origin to the 
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