OUR DUTY TO THE FUTURE 587
wateiy and its sudden and unexpected attacks. The past has wit- nessed many an ancient work mined or damaged by water^ and here also the hand of nature was seen more often than that of man. On the other hand^ destruction by fire has doubtless been occasioned much more often by man than by nature. Whether started by accident or by design, fire has ever been a terror to all peoples. While many ancient works were of a character to remain to a large extent unafEected by fire, yet some of the things most desirable for us to have known about were utterly lost, being readily combustible. It is certainly fortunate for us that such a large number of written records were made on stone, clay or metal instead of on so combustible a substance as papyrud. What would we not now give for manuscripts long since destroyed, as those in the famous library at Alexandria, or the Aztec writings burned by fanatical Spaniards? We are able to see, then, how these agents, the primitive elements, air, water and fire, have affected man's works in the past, and what may be expected of them in the future.
Still other natural forces deserve our attention. Among the most prominent of these are the two extremes, glacial and desert conditions. The destructive action of ice and snow is to be seen somewhere almost every season on a relatively small scale. But what of a frigid epoch like that of the Pleistocene when half a continent was covered with im- mense ice sheets ? We only have to be reminded that in those times the destruction of nature's own works was enormous, in order to realize what might have happened to ancient works, and what may yet happen to our own works of to-day. Whether acting as glacier, iceberg or floe, ice must be reckoned with as a possible natural agent in destroying the so-called imperishable" habitations and monuments of man. And with ice we must put the avalanche, a swifter but less powerful ally. One might also speak here of landslides, an agent of destruction that is not at all unfamiliar in many parts of the world.
The desert areas of the globe afiford us some excellent examples of the effect of torrid and arid conditions upon man's works. The deserts are quite frequently areas of violent atmospheric disturbances with al- most an entire lack of precipitation and consequent dearth of vegeta- tion. The soil is sand, or alkali, or a mixture of these, and the winds keep the small dry particles in almost constant motion. Eock sur- faces are soon cut, etched, and eroded to an astonishing degree by the force of nature's sand-blast. Is it surprising that there are but few inhabitants, and these mostly nomadic, in the desert places? Yet on the sites of several present desert regions there flourished in ancient times civilized peoples whose only relics that remain to us are a few ruins of stone buildings, a few mummies, some specimens of pottery, and a few metallic ornaments. The present Sahara desert was in recent geologic times the home of animals that subsisted upon abun- dant vegetation. The Desert of Gobi has overtaken and concealed cities
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