Parr I. Sect. iv. § 3.]. EFFECTS OF PRESSURE. 309 
linked together with those of igneous action as connected manifesta- 
tions of hypogene change. ‘he author has further suggested a 
relation between periods of extensive metamorphism and periods 
of yoleanic eruption. He has pointed out that in the geological 
history of Britain there are indications of such a relation, the volcanic 
eruptions of the Old Red Sandstone period, for example, succeeding 
the time when the Silurian rocks of the Scottish Highlands were 
crumpled and metamorphosed.’ 7 
§ 3. Effects of pressure. 
Besides the influence of pressure in raising the melting point of 
rocks, and in permitting water to remain fluid among them at 
very high temperatures, we have to consider the effects produced 
by the same cause upon rocks already solidified. _The most 
obvious result of pressure is consolidation, as where a mass of 
loose sand is gradually compacted into a more or less coherent 
stone, or where, with accompanying chemical changes, a layer 
of vegetation is compressed into peat, lignite, or coal. The 
cohesion of a sedimentary rock may be due merely to the pressure 
of the superincumbent strata, but some cementing material has 
usually contributed to bind the component particles together. Of 
these natural cements the most frequent are peroxide of iron, silica, 
and carbonate of lime. 
Pressure equally distributed over a rock presenting everywhere 
nearly the same amount of resistance will promote consolidation, 
but may produce no further internal change. If, however, the. 
pressure becomes extremely unequal, or if the rock subjected to it 
ean find escape from the strain in one or more directions, there may 
be a rupture in the continuity of the mass or a rearrangement of 
its particles, which by this means are made to move upon each 
other. Five consequences of these movements may be briefly 
alluded to here in illustration of hypogene action in dynamical 
geology. A fuller account of their effects in the general structure 
of rock-masses will be found in Book IY. 
(1.) Minor Ruptures and Noises.—Among mountain valleys, 
in railway tunnels through hilly regions, or elsewhere among rocks 
subjected to much lateral pressure, sounds as of explosions are 
occasionally heard. ‘These noises are probably the result of relief 
from great lateral compression. The rocks have for ages been in a 
state of strain, from which, as denudation advances, or as artificial 
excavations are made, they are relieved, and this relief takes place, 
not always uniformly, but sometimes cumulatively by successive 
3. Bo ee 
PT Mw 5 
shocks or snaps. Mr. W. H. Niles of Boston has described a number 
of interesting cases where the effects of such expansion could be seen 
in quarries; large blocks of rock being rent and crushed into 
fragments, and smaller pieces being even discharged with explosion 
’ Trans, Geol. Soc. Edin. ii. p. 287. 
