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286 DYNAMICAL GEOLOGY. 

; 45,000,000 years. We must either therefore multiply enormously 
- the periods required for geological changes, or find some cause which ~ 
could have elevated great mountain-chains at more recent intervals. 
But it is well worth consideration whether the cause suggested by 
Mr. Darwin may not have given their initial trend to the masses of 
land, so that any subsequent wrinkling of the terrestrial surface due 
to any other cause would be apt to take place along the original lines. 
To be able to answer this question it is necessary to ascertain the 
dominant line of strike of the older geological formations. But 
information on this subject is still scanty. In Western Europe the- 
revalent line along which terrestrial plications took place during 
Be Aecanaic time was certainly from 8.W. or 8.8.W. to N.E. or N.N.E., 
and the same direction is recognizable in the eastern States of North 
America, But the trend of later formations is more varied. The 
striking contradictions between the actual direction of so many 
mountain-chains and masses of land, and what ought to be their line 
according to the theory, seem to indicate that while the effects of 
internal distortion may have given the first outlines to the land areas 
of the globe, some other cause must have been at work in later times, 
acting sometimes along the original lines, sometimes transverse to 
them. 
The main cause to which geologists are now disposed to refer the 
corrugations of the earth’s surface is secular cooling and consequent. 
contraction. If our planet has been steadily losing heat by radiation 
into space, it must have progressively diminished in volume. The 
cooling implies contraction. According to Mr. Mallet, the diameter 
of the earth is less by at least 189 miles since the time when the ~ 
planet was a mass of liquid.’ But the contraction has not manifested 
itself uniformly over the whole surface of the planet. ‘The crust 
varies much in structure, in thermal resistance, and in the position 
of its isogeothermal lines. As the hotter nucleus contracts more 
rapidly by cooling than the cooled and hardened crust, the latter 
must sink down by its own weight, and in so doing requires to 
accommodate itself to a continually diminishing diameter. The — 
descent of the crust gives rise to enormous tangential pressures. 
The rocks are crushed, crumpled and broken in many places. 
Subsidence must have been the general rule, but every subsidence 
would doubtless be accompanied with upheavals of a more limited 
kind. The direction of these upheaved tracts, whether determined, — 
as Mr, Darwin suggests, by the effects of internal distortion, or by 
some original features in the structure of the crust, would be apt to 
be linear. The lines, once taken as lines of weakness or relief from 
the intense strain, would probably be made use of again and again 
at successive paroxysms or more tranquil periods of contraction. 
Mr. Mallet has ingeniously connected these movements with the 
linear direction of mountain chains, volcanic vents and earthquake 
shocks. If the initial trend to the land-masses were given as hypo- 
1 Phil. Trans, 1873, p. 205. . 4 
