OUR DUTY TO THE FUTURE 59»
are? Is there not a possibility of their shifting^ very gradually per- haps, but nevertheless with a distinct movement, noticeable every fifth or tenth century ? The question is one that can not be definitely solved. The belts will doubtless move together, but can we say that any given portion of the earth's crust will be forever free of earthquake or vul- canism?
Man seems to delight in building his cities as near to danger as possible. Witness the manner in which the peoples of the Mediter- ranean insist on staying by their volcanoes, and rebuilding their shat- tered cities. Taking an example at home, may not San Francisco run the risk, with other coast cities, of being destroyed once more, or pos- sibly several times, by earthquake? The archeologists of the future may find several buried cities on the present site of San Francisco, if man insists on living at that point as the Pacific coast littoral becomes broader, and the Sierras rise higher to the accompaniment of imnum- bered quakings and tremblings.
But even though stone, the favorite building material since prehis- toric times, is able to survive or be spared the effects of earthquake, fire, vulcanism and fiood, it is still subject to final and complete disin- tegration through the slow but continued action of the weather. Wind, rain, frost, the gases of the air, and even the humblest living organisms combine to overthrow that which man has erected, and which may have withstood all other destructive agents. In this respect granite is almost as unstable as the other varieties of building stones mentioned. The coarser the grains in a granite rock, the more easily the rock crumbles. It is readily split or fractured by frost, by trees, and even by the heat of the sun. Limestones and marbles are slowly dissolved by the carbon dioxide in rain and in percolating waters. If in contact with soils and vegetation, a similar effect is observed, due to the same cause. Sandstones may resist the effect of frost, rain, and carbon dioxide, but succumb at once to the chiseling effect of sand particles driven by the wind. Eunning water is often more active than wind, rain or air in disintegrating and pulverizing rocks; while a glacier either moves bodily whatever comes in its path, or rides over it and crushes it to the finest powder. After reviewing thus the effects of the principal de- structive agents upon stone, to put our faith in stone as the sole protec- tion to our treasures of the past and present would seem to be open to several objections.
Next to stone, we find man has discovered and placed a value upon certain substances which he has learned to call metals. At the present time we may be said to be living in the age of steel. Iron is our most important metal as it is the most useful and most abundant, or rather, is the cheapest and is most readily obtained in large quantities. Yet iron is the most perishable of all metals. Eust is the chief enemy of
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