THE MEDITERRANEAN NATURALIST 
the lava was sufficiently viscous. Neither should 
we expect such an exudation of fluid rock to be 
accompanied by mechanical vibrations other than 
that dependent upon the formation of the fissure, 
or duct, by which the lava escaped, and which 
formation would be dependent upon causes extran- 
eous to the actual expulsion of the fluid magma. 
That such favourable conditions may sometimes 
occur, so that the actual dyke may traverse strata 
that are not water-logged, we cannot deny, and 
possibly some of the great basalt plains of America 
and elsewhere may so have originated; yet, geo- 
logy teaches us to consider such to be rather the 
exception than the rule. 
Intrusion of Igneous Matter into Dry, or nearly 
Dry, Docks, hut not reaching the Surfa.ce.— -tShould 
a fissure opening downward to the volcanic magma 
be formed by secular cooling or other means, we 
should expect that it would be simultaneously 
filled by the oozing in of the igneous magma. This 
mass of fused silicates, at a very high tempera- 
ture, will now undergo a series of changes, which 
we will attempt to trace. The first thing will be 
the cooling of a layer of the magma, which is in 
actual contact with the walls of the fissure; and 
should that substance be in a purely vitreous 
condition, a pitchstone salband of variable thick- 
ness will result. Now, should the conductivity of 
the surrounding strata be great, or should the 
temperature of the magma be near solidification 
point, then that process will continue from the 
salband inwards through the whole mass, and a 
blind dyke will result. On the other hand, should 
the surrounding strata be bad conductors, already 
heated, and the magma at a very much higher 
temperature than that of its solidification, so that 
its heat might be given out quicker than the 
surrounding rocks could absorb, any salband that 
might at first have been formed would be refused, 
and such re-fusion might extend some distance 
into the surrounding rocks, continuing to do so 
until the supply of heat of the injected material 
was exhausted. Should the surrounding rocks be 
infusible, a chemical interchange would take 
place between the igneous and solid matter, 
resulting in the metamorphism of the former, and 
a corresponding change in the latter. Although 
I am not personally acquainted with many exam- 
ples in illustration of this condition, probably | 
1.93 
some of those dykes which are so abundant in the 
Western Isles and Highlands of Scotland, described 
by Jameson and others who have followed him, 
will serve. If the intrusion of the igneous magma 
takes place in solid rocks, which themselves are at 
a high temperature from pressure, crushing or con- 
duction of heat upwards from below — three things 
will probably result. First, the magma, from the 
small absorption of its heat by the surrounding 
rocks, would require a very long time to cool, and 
that would also occur in a very gradual and uniform 
manner, so that an extremely coarse crystalline 
structure would result. This is the case in a great 
number of pegmatite granite veins. Secondly, no 
salband will be formed, and partial fusion of the 
fissure walls may occur, so that in gneisose rocks 
the line of demarcation between them and the 
intrusive granite, or syenite, may be very ill- 
defined. Thirdly, the condition will be highly 
favourable to contact metamorphisrn, which, in 
such cases, often extends into the surrounding- 
rocks for very considerable distances, often many 
hundreds, or even thousands, of yards. (1) Jukes 
maintained that the granite forms the basis of 
many volcanoes, being the source of the eruptive 
matter. It has beer, observed by Cotta, that the 
smaller the dyke the smaller the grain, (2) which 
is explained by the more rapid cooling of the 
smaller mass. I have seen many examples illu- 
strating this point in the Tyrol. We have the 
same in nearly all kinds of dykes, where the 
nearer we approach the outer surface the finer 
the grain; though volcanic dykes in cones are 
an exception for some minerals. Negri and 
Spreafico, in describing an expansion of por- 
phyry near Lugano, show that the felspars near 
the surface are invisible, so that the rock is a 
euritic porphyry. Towards the centre of this 
great mass the crystals are distinct, but round and 
imperfectly formed, whilst in the dyke, which 
supplies this great mass, the crystals are very 
perfect and large, often reaching three centimeters 
in diameter. We see, therefore, that the perfection 
of crystallization, and the type of resulting rock, 
are in direct relation with the length of time and 
quietness of the cooling of the magma. In the 
same way we may explain the crystals in the 
( 1 ) L. Gatin, Yulccniismo, 1885, y. 28. 
(2) Naumann % Lehr buckler Geologic, 1858-1808. 
