But subsidence of the ocean bed has another and different effect, which causes the appearance of dry land without upheaval. The world is continually shrinking in diameter at the rate of about four feet in a year. The sun’s present rate of contraction, as determined by astronomers, is four miles in a century, that of the earth is one mile in about fourteen centuries.
Every volcanic outburst, every earth tremor, is the effect of this shrinking of the earth’s diameter, and there is no outpouring of volcanic production that is not accompanied by a compensating subsidence at some part of the earth’s surface. But as the area of the oceans is two and a half times the area of dry land, it follows that five-sevenths of the subsidences which take place are in the ocean, and, therefore, not visible to us, while much of that which takes place even on the land is in inaccessible and uninhabited parts, of which we have no cognisance. Thus we know of but a very small proportion of the subsidences which are continually taking place, although the seismograph, an invention of recent years, places on record very many earth tremors of which we otherwise would have no knowledge. All these tremors are the result of the settlement of the earth’s crust, to accommodate itself to the continually altering strain of which volcanic outbursts are the visible effect.
These subsidences of the ocean bed, drain the waters off the shallower portions, or oceanic plateaux, such as that on which the Abrolhos stand. If the highest of these coral islands had been built up to the water level, and a series of subsidences in various parts of the ocean had taken place which lowered the surface of the waters, say, twelve feet, that would leave those islands standing twelve feet out of the water; while those portions of the plateau on which the coral insects had only built to within eight feet of the surface, would be left four feet out of the water, and the relative levels of the islands would thus remain unaltered.
One cannot sail amongst the Abrolhos group without being struck with the unaltered relative positions in level which they occupy, and, therefore, that their existence to-day is due to subsidences in the ocean’s bed which have taken place in various parts of the world, probably at various periods, to tell us of seismic disturbances of which we otherwise have no record.
TRANSCRIBER’S NOTES:
Obvious typographical errors have been corrected.
The text on page 26 from Pinafore has been retained from the original, although the actual text of the second line is “And seek the seclusion that a cabin grants.”