Compensation of Stress and Dislocation
In both cases, subsidence and elevation, such processes are always accompanied by breaking strata and the formation of cleavages, that are the real and direct causes of earthquakes and their undulating propagation. In the volcanic elevation areas of South America such processes can be reconstructed by analyzing the position of strata. With his theoretical considerations DARWIN continues a treatise by HOPKINS in the Philosophical Transactions.[1] In this treatise HOPKINS had proved that the first effect of a regular elevation of longitudinal section of the crust is the formation of parallel cleavages that are parallel to the longer axis, as shown in a figure by HOPKINS that was used by DARWIN: (figure 32a)

When these rectangular independent pieces lower again, this happens in an irregular form, unlike the elevation process. The blocks are deposited in a very irregular way resulting at best in the following positions as a figure by HOPKINS shows (figure 32b):

In the cordilleras, however, the strata are usually inclined more than 45 degrees, sometimes they stand vertically. Thus DARWIN asks

"how is it possible, that some of the masses of strata should be placed vertical, and others absolutely overturned, by the action of the fluid rock, without the very bowels of the earth gushing out".[2]
DARWIN's answer is that this elevation is not a single violent event but a slow gradual process of elevation with time enough for the cleavages and faults reaching up to the surface to fill with liquid rock and then cool down. A volcano is formed only where the formation of cleavages is covering a whole area of the crust:

"In this manner the strata, each new fracture being firmly cemented by the cooling of the injected rock, might be overturned into any possible position, and yet, from a gradually thickening crust being formed over the fluid mass, on which the whole is believed to rest, the earth would be protected from a deluge of lava. If this reasoning be sound, we may deduce this remarkable conclusion, that in a mountain-chain, having an axis of plutonic rock, which was propelled upwards in a fluid state, where the strate betray the effects of the most violent action, although it be on a gigantic scale, there we have the best evidence of an almost infinite series of small movements."[3]
As an explanation DARWIN adds another figure, taking over from the drawing by HOPKINS the length of the parts of the strata but increasing their diameter. Thus it becomes clear that the masses cannot be forced back into their previous horizontal limits. The compression of such huge pieces is caused by an enormous disarrangement of the strata that takes place in many small steps and not suddenly and violently (figure 32c).

DARWIN indicates explicitly which tradition his argumentation belongs to:

"This whole view is nothing more than an application of Hutton's doctrine of the repetition of small causes to produce great effects; and which Mr. Lyell has already brought distinctly to bear on this particular subject."[4]
DARWIN himself summarizes the charactistic elements of his plutonistic earthquake theory by far transcending those of his predecessors due to his own experience and research:

"From these considerations, we may, I think, fairly conclude, with regard to the earthquakes on the west coast of South America,
1st. That the primary shock is caused by a violent rending of the strata, which seems generally to occur at the bottom of the neighbouring sea.
2nd. That this is followed by many minor fractures, which, though extending upwards nearly to the surface, do not (excepting in the comparatively rare case of a submarine eruption) actually reach it.
3rd. That the area thus fissured extends parallel, or approximately so, to the neighbouring coast mountains.
4th. That when the earthquake is accompanied by an elevation of the land in mass, there is some additional cause of disturbance."[5]
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