98 
SUBTERRANEAN VOLCANIC ACTION 
BOOK I 
where the magma has the least amount of overlying load, and encounters 
that relief from pressure which enables it to become a powerful agent in 
terrestrial physics. 
But in the case of the larger bodies of injected rock, especially where 
they do not seem to have been accompanied by the opening of any volcanic 
vents, the propulsion of the igneous material into the crust has probably been 
effected as a consequence of disturbance of the terrestrial crust. When the 
strain of contraction leads to the pushing upward of the terrestrial areas 
intervening between wide regions of subsidence, even though the differential 
movement may be slight, the isogeotherms undergo deformation. The 
intensely hot nucleus is srpreezcd upward, and if in the process of compres- 
sion ruptures take place in the crust, and cavities in it are consequently 
opened, the magma will at once be forced into them. Such ruptures may 
be expected to take place along lines of weakness. Bocks will split along 
their stratification-planes, and the tendencv to separation along these lines 
may be aided by the readiness of the energetic magma to find its way into 
and to enlarge every available opening. Hence we may expect that, besides 
vertical fractures, leading to the production of dykes and bosses, there will 
often be horizontal thrusts and ruptures, whicli will give rise to the forma- 
tion of sills.’ 
There is still another feature of terrestrial contraction which may help 
us to follow the behaviour of the magma within the crust. Plication of the 
crust is one of the most characteristic resixlts of the contracting strain. 
Where a great series of sedimentary formations has been violently compressed 
so that its component strata have been thrown into rapid folds and squeezed 
into a vertical position, the portion of the crust thus treated may possibly be 
on the whole strengthened against the uprise of molten material throiigh it. 
But the folding is often accompanied with dislocation. Not only are the 
rocks thrown into endless plications, but portions of them are ruptured and 
even driven horizontally over other parts. Such greatly disturbed areas of 
the crust are not infrequently found to have been plentifully injected with 
io-neous rocks in the form of dykes, veins, sills, laccolites and bosses. 
The elevation of a mountain-chain is known to be accompanied with a 
diminution of density in the crust underneath. Mr. 0. Fisher has sug- 
gested that along such lines of terrestrial iiplift there may be a double 
bulge in the crust, one portion rising to form the upheaved land and the 
other sinking down into the hot nucleus. If the lighter descending crust 
were there melted it might form a magma ready to be poured out as lava 
on the opening of any vent. The lava thus ejected would be of the lighter 
kinds. It has been remarked as certainh^ a curious fact that the lavas 
which issue from high mountain ranges are generally much more acid than 
the lieavy basic lavas which are so characteristic of volcanoes close to the 
level of the sea. 
But even wdiere no actu.al mountain-chain is formed, there are gentle 
undulations of the crust wdiich no doubt also affect the isogeotherms. If 
any series of disturbances should give rise to a double system of such 
