SEPTEMBER 18, 1902] 
the hot springs always appear at the very edge of the river, 
whatever may be the height of its waters during drought or 
flood. The statement is probably true, for I think the springs 
well up from below through the walls of a fault that traverses 
the bed of the Satlej at a high angle to its course, and the 
springs thus come to the surface on both its banks. 
The metamorphic influence of these springs on the rocks in 
this locality has been very powerful. The ancient volcanic 
rocks there exposed have, for some distance up the river, been 
altered by aqueous agents almost out of recognition. The 
original structural characters of these lavas have been almost 
completely broken down and an amorphous substance substituted 
for the crystals and minerals of which they were originally 
composed. 
This result shows that the crystals and minerals of these old 
lavas must, for all practical purposes, have been completely 
porous to the aqueous agents brought to bear on them. 
The general transmutation of one mineral into another by 
the action of heated water holding mineral agents in solution, 
aided by heat and pressure, may take place in a variety of ways. 
Some of these processes are simple, but others are highly com- 
plex. Many are the results of a single operation, others of a 
series of changes, some of which prepare the way for those that 
follow. 
In some cases the change may be brought about by the re- 
moval, in whole or in part, of one or more of the essential 
constituents of a mineral, whereby the relative proportions and 
mutual relations of those that remain are altered, as the 
following examples will show. 
By loss of water limonite passes into hematite, and opal into 
crystalline quartz. Dyscrasite, by loss of antimony, passes into 
native silver, and pyroxene, by the removal of its lime and iron, 
is changed into talc, Simple oxidation or the absorption of 
oxygen bya mineral is responsible for another class of changes, 
as in the conversion of zinc blende into goslarite, and antimony 
into valentinite. 
The loss of one or more of the ingredients, concurrently with 
the introduction of one or more new ones, causes many meta- 
morphic changes, as in the conversion of marcasite into 
magnetite, of witherite into barite, and of azurite into 
malachite. 
The well-known conversion of a peridotite into serpentine is 
a case in point. Here, part of the iron and magnesia is re- 
moved from the olivine, and water is introduced. A simple 
process like this, brought about by the percolation of surface 
waters through an igneous rock, is sufficient to transform con- 
siderable areas of rock masses into serpentine, as has been the 
case in parts of Cornwall. 
Some metamorphic processes are more complex than those 
alluded to above, but Nature has unlimited time at her disposal, 
and is able to manufacture potent chemical reagents as her pro- 
cesses proceed. For instance, the sulphides of various metals 
of common occurrence in rocks, most of which, with the excep- 
tion of those of the alkaline metals, are insoluble in water, by 
taking up oxygen pass into sulphates, most of which are 
soluble in that liquid at the ordinary temperature. 
These sulphates are readily carried away in solution, and be- 
come potent factors of change in rocks through which water 
charged with these salts flows. Again, carbon dioxide, so 
abundant in percolating water, decomposes minerals containing 
lime or alkali, and removes them as soluble carbonates to effect 
powerful chemical reactions elsewhere. 
I must pass over the subjects of paramorphism and pseudo- 
morphism, as the limited time at iny disposal does not permit 
me to enter upon these subjects. 
In the above sketch I have contented myself with a brief dis- 
cussion of some of the leading principles that seem to me to 
underlie contact action and metamorphism in the wet way, be- 
cause I venture to think that, if we really understand these two 
divisions of our inquiry, it will be unnecessary on the present 
occasion to enlarge on other branches of our subject. 
Take, for instance, what is commonly called dynamic meta- 
morphism. The main factors in this kind of metamorphism 
are the folding, crumpling, crushing and shearing of rocks by 
earth movements, especially during the upheaval of mountains. 
But these dynamic forces are potent factors in the develop- 
ment of heat. 
In the case, therefore, of dynamic metamorphism, as in con- 
tact metamorphism, pressure and heat are the main factors 
acting in conjunction with the water shut up in or circulating 
NO. 1716, VOL. 66] 
NATURE 
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through a rock. If we understand how these factors operate 
and produce the results we see in cases of contact metamorphism, 
we shall not fail to understand their action in a case of dynamic 
metamorphism. 
These observations also apply to regional metamorphism ; 
that is to say, to metamorphism produced in rocks at great 
depth, by being brought within the influence of the interior heat 
of the earth. The action of heat in increasing molecular 
motion and kinetic energy is well understood nowadays, and so 
long as we get heat it seems to me immaterial how heat is 
generated in rocks subject to metamorphic action. 
In the above sketch I have intentionally omitted to enter 
into the details of chemical and mineralogical action that have 
brought about individual cases of metamorphic change. 
Volumes would be required to do justice to so complex a sub- 
ject, and the details would, in an opening Address, be out of 
place. 
In conclusion I have, I trust, shown how important a part 
water plays as an agent of metamorphism, not only at and 
near the surface of the earth, but at plutonic depths. We have 
seen that the molten granite of the Satlej Valley, which was 
given as an illustration of a fluid igneous magma, contained a 
considerable proportion of water held in solution at considerably 
above red heat, and that the fluidity of the magma was due to 
its presence. We also saw that the great heat to which the 
magma was raised increased the potential energy of the con- 
tained water when a relief of pressure opened the way for the 
intrusion of the molten magma into neighbouring rocks. We 
also saw that this water was rendered by heat a powerful 
solvent, and that it carried with it into the adjoining rocks the 
mineral matter of the granite in solution. We also saw that 
heat increased the porosity of minerals, facilitated the passage 
of liquids laden with mineral matter through their pores, and 
increased the potency of chemical action. 
SECTION F. 
ECONOMIC SCIENCE AND STATISTICS. 
OPENING ADDRESS BY EDWIN CANNAN, M.A., LL.D., 
PRESIDENT OF THE SECTION. 
Ir it happened every year that the President of this Section 
undertook to justify his own existence, I am afraid the Section 
would become weary. But my four distinguished predecessors 
have all been drawn from the Civil Service, and though each of 
us may have doubts about particular branches of the Civil 
Service, we are mostly willing to allow that as a whole it is at 
Jeast a necessary evil, so that we do not get apologies from the 
Presidents who, so to speak, represent the practice of political 
economy. I hope, therefore, that you will bear with me if I 
offer some reasons for thinking that the teaching and study of 
the theory of economics is not, as many people seem to suppose, 
a wholly unnecessary evil, but, on the contrary, a thing of very 
great practical utility. 
I do not mean to argue that a knowledge of economic theory 
will enable a man to conduct his private business with success. 
Doubtless many of the particular subjects of study which come 
under the head of economics are useful in the conduct of busi- 
ness, but I doubt if economic theory itself is. It does not indeed 
in any way disable a man from successful conduct of business ; 
I have never met a decent economist who was in a position of 
pecuniary embarrassment, and many good economists have died 
wealthy. But economic theory does not tell a man the exact 
moment to leave off the production of one thing and begin that 
of another ; it does not tell him the precise moment when prices 
have reached the bottom or the top. It is, perhaps, rather 
likely to make him expect the inevitable to arrive far sooner 
than it actually does, and to make him underrate, not the fore- 
sight, but the want of foresight of the rest of the world. 
The practical usefulness of economic theory is not in private 
business, but in politics, and I for one regret the disappearance of 
the old-name ‘‘ political economy,” in which that truth was 
recognised. 
One of the commonest complaints of the time is that there is 
no text-book of economics which commands any really wide 
approval, and you may therefore, I think, fairly ask me to ex- 
plain what I mean by the teaching and study of economic theory 
before I undertake to prove its practical usefulness in the 
discussion of legislative and administrative measures. I will 
