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waters contain in solution an amount sufficient to increase their specific gravity by 2.7 per cent.

Taking the last point first, it is interesting to note the effects upon the bulk of the ocean which has resulted from the matter dissolved in it.  From the known density of average sea water we find that 100 ccs. of it weigh just 102.7 grammes.  Of this 3.5 per cent. by weight are solids in solution.  That is to say, 3.594 grammes.  Hence the weight of water present is 99.1 grammes, or a volume of 99.1 ccs.  From this we see that the salts present have increased the volume by 0.9 ccs. or 0.9 per cent.

The average depth of the ocean is 2,000 fathoms or 3,700 metres.  The increase of depth due to salts dissolved in the ocean has been, therefore, 108 feet or 33.24 metres.  This result assumes that there has been no increased elastic compression due to the increased pressure, and no change of compressional elastic properties.  We may be sure that the rise on the shore line of the land has not been less than 100 feet.

We see then that as the result of solvent denudation we have to do with a heavier and a deeper ocean, expanded in volume by nearly one per cent. and the floor of which has become raised, on an average, about 700 feet by precipitated sediment.

One of the first conceptions, which the student of geology has to dismiss from his mind, is that of the immobility or rigidity of the Earth’s crust.  The lane, we live on sways even to the gentle rise and fail of ocean tides

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around the coasts.  It suffers its own tidal oscillations due to the moon’s attractions.  Large tracts of semi-liquid matter underlie it.  There is every evidence that the raised features of the Globe are sustained by such pressures acting over other and adjacent areas as serve to keep them in equilibrium against the force of gravity.  This state of equilibrium, which was first recognised by Pratt, as part of the dynamics of the Earth’s crust, has been named isostasy.  The state of the crust is that of “mobile equilibrium.”

The transfer of matter from the exposed land surfaces to the sub-oceanic slopes of the continents and the increase in the density of the ocean, must all along have been attended by isostatic readjustment.  We cannot take any other view.  On the one hand the land was being lightened; on the other the sea was increasing in mass and depth and the flanks of the continents were being loaded with the matter removed from the land and borne in solution to the ocean.  How important the resulting movements must have been may be gathered from the fact that the existing land of the Globe stands at a mean elevation of no more than 2,000 feet above sea level.  We have seen that solvent denudation removed over 1,600 feet of rock.  But we have no evidence that on the whole the elevation of land in the past was ever very different from what it now is.

We have, then, presented to our view the remarkable fact that throughout the past, and acting with extreme