506 
—by measuring the angles at which they extinguish from the 
twinning or the pinacoidal plane. 
This is not mere theory. Each species of felspar has its 
own angle of extinction and its own index of refraction. The 
determination of these two factors enables a petrologist to prove 
optically the change in composition ; or, in other words, the 
change in species which has taken place in the successive zones, 
during the gradual growth of a large zonal felspar. 
Another general rule must now be mentioned. I think it may 
safely be asserted as a broad rule that the different species of 
felspars are attackable by the chemical reagents which make 
themselves felt in metamorphic action, in the order of their 
basicity ; that is to say, the more basic felspars are more easily 
attacked than theacid ones. When we bear in mind the facts 
stated above, we shall, I think, be able to see clearly how it is 
that the peripheral portions of large felspars in igneous 
rocks sometimes escape alteration, whilst the cores of these 
crystals are converted into secondary minerals, such as chlorite, 
silvery mica, zoisite, epidote, kaolin, steatite, saussurite, calcite 
and scapolite, 
The chemical reagents flowing in solution through the pores of 
the felspars, pass by the more acid and refractory species and 
devote their energies to the more susceptible basic species en- 
tombed atthe heart of the zonal crystals. 
The point I wish to enforce most strongly is that the phen- 
omenon above described, namely, the formation of secondary 
metamorphic minerals in the interior of a crystal, combined 
with the comparative immunity to change of the external por- 
tions, shows that the agents which brought about chemical 
changes at the core of the crystal flowed freely through its 
unaltered peripheral portions. 
But some may ask whether the chemical agents referred to 
may not have gained access to the heart of a crystal bya crack. 
I answer that a crack is a coarse and tangible object that looms 
large under the microscope. A crack in a mineral liable to 
metamorphic action, through which chemical reagents have 
flowed, could not escape detection. The finest crack through a 
homogeneous mineral, such as, for instance, an olivine, can be 
readily seen, not only by the small canal worn by the corrosive 
action of the chemical agents that flowed through it, but by the 
alteration set up in the mineral along the whole course of the 
canal. 
I have a thin slice from a beautifully fresh olivine contained 
in one of the lavas of Vesuvius collected by myself. A volcanic 
explosion or other cause, operating after the crystallisation of the 
olivine, produced a very fine crack in the mineral through which 
water, charged with chemical reagents, subsequently flowed. 
The crack, though of microscopic width, is filled with serpen- 
tune, and on both margins fibrous serpentine has been formed 
at the expense of the parent olivine, and constitutes a fibrous 
band on both sides of the crack throughout its entire length, 
the direction of the fibres being at right angles to the crack. 
The rest of the olivine is of virgin purity and polarises in the 
most brilliant colours, contrasting strongly with the serpentine. 
In this case it is clear that the chemical reagents, through free 
to flow along the crack, had commenced to extend beyond its 
walls, encouraged thereto by the porosity of the olivine itself. 
But how different is this case from those in which the entrance 
of the chemical agents had not been facilitated by a crack. In 
the case above described, the chemical changes set up were 
limited to the borders of the crack, and even had they gradually 
extended in the course of time to the whole of the olivine, the 
original canal by which the chemical.reagents had gained access 
to the crystal would have remained to tell its tale, and exhibit 
along its course the banks of iron oxide thrown down by the 
chemical navvies that had excavated it. 
Cracks save time as roads and canals do, but they leave be- 
hind them evidence of their former existence. In order to 
understand fully how rocks and minerals are so completely 
open to the attacks of chemical reagents, which penetrate to 
and produce chemical and mineralogical changes at the very 
hearts of minerals, we must fully realise how completely porous 
rocks and minerals are, to the heated liquids which carry these 
reagents with them in solution. Heat, as before stated, not 
only increases chemical energy, but destroys more or less com- 
pletely the cohesion between molecules, and increases the 
amplitude of the vibrations, or other motions of the mole- 
cules, and consequently facilitates the entrance of liquids and 
gases into the pores of minerals, and their complete permeation 
by these powerful agents of change. Thus far we have been 
NO. 1716, VOL. 66] 
NATO 
[SEPTEMBER 18, 1902 
chiefly concerned with some of the principles underlying the 
branch of our subject embraced by the term contact meta- 
morphism, which implies operations conducted at considerable 
depths below the surface of the ground, under conditions of 
heat and pressure. 
We must now consider very briefly changes produced at or 
near the surface by the agency of water, or, as Bischof in his 
well-known work termed it, metamorphism in the ‘‘ wet way.” 
No hard-and-fast line, however, can be drawn between the 
two classes of operations, as the one gradually shades by fine 
gradations into the other. Atone end of the scale we have high 
pressure and high temperature, and a fluid igneous magma hold- 
Ing water in solution, above a red heat, and giving up heated 
water or vapour charged with salts to the rocks in contact 
with it. 
Passing to the other end of the scale through diminishing 
temperatures and pressures, we reach a condition in which the 
water circulating through the rocks at ordinary pressure and 
temperature is more abundant in amount, and holds acids and 
salts in solution, capable of setting up important chemical re- 
actions in the rocks and minerals to which it gains access. 
In the case of surface operations, moreover, the metamorphic 
agents—water, acids, salts—are being constantly renewed. 
Conditions differing as widely as the conditions at the extreme 
ends of our scale do not yield, however, precisely the same 
results. In both metamorphic change goes on with more or less 
briskness, but the products are different. Some minerals re- 
quire great heat and great pressure for their production, and 
such minerals are never formed by any surface process of 
weathering. For instance, the temperature reached determines 
whether titanium dioxide crystallises as rutile, or in one of its 
other two forms, rutile requiring a temperature of more than 1000” 
C., and being the only form of titanium dioxide “stable at a high 
temperature.” 
Temperature§ also seems to determine whether the silicate of 
alumina crystallises as andalusite, kyanite or sillimanite, the 
two former being transformed into the latter, at a temperature of 
1320° C. to 1380° C. 
On the other hand, some minerals require little heat for their 
formation, and are readily produced by metamorphic changes 
in the ‘‘ wet way.” 
There seems to be some correspondence between the melting 
point of minerals and their density ; thus in the case of eleven 
minerals produced by contact metamorphism, whose average 
specific gravity ranges from 3°06 to 4’03, I find that their melt- 
ing point ranges from 954° to above 1770° C., high temperature 
and high pressure (a concomitant of plutonic conditions) appear- 
ing to be factors in the production of high specific gravity in 
minerals. 
The genesis of individual species of minerals is a fascinating 
study, but the subject is too large to enter upon here. 
Water gains access to rocks in several ways. It falls as rain ; 
it rises from hidden depths ; it leaks from the sea into horizontal 
beds or into strata dipping away from it; and it penetrates 
through faults and fissures. Rain in its descent takes up from 
the air oxygen, nitrogen, carbonic acid, and in some cases small 
amounts of nitric acid. 
It is thus in itself a powerful solvent and potent agent in 
producing chemical change. 
In its passage through the surface soil it dissolves humic and 
other organic acids, the products of vegetable decay, which add 
greatly to its solvent power and enable it to break up many 
silicates and to dissolve even silica. 
By the time the rain-water reaches the solid rocks below the 
surface soil, it has become a very active agent in producing 
chemical change in them. It is by such agents, persistently 
applied during long periods of time, that large areas of ultra- 
basic igneous rocks have been altered into serpentine. 
Hot springs are a well-known instance of water rising in 
considerable quantity from plutonic depths. They are known 
to occur in the plains of India, and are especially abundant in 
the Himalayas. I visited two very interesting ones at Suni, in 
the bed of the Satle} River, west of Simla. These springs 
rise apparently under the very bed of the river, and come to the 
surface on both banks within a yard or two of the rushing water 
of the Satlej. When I visited the springs they had a tempera- 
ture of 130° F., and contrasted strongly with the cold water of 
the river flowing past them, which had descended from high 
Himalayan glaciers and had a temperature of 49° F. 
The native nhabitants of neighbouring villages told me that 
