Page:Popular Science Monthly Volume 69.djvu/89

Rh Cracks were formed by the earthquake in each locality. In San Francisco except along the immediate line of faulting they were few in number, small in size, and limited to small tracts of especially soft ground or to the steeper hillsides. In Charleston they occurred over an area several miles in diameter, but were usually under an inch across, except near the rivers. At New Madrid, on the other hand, they extended over an area many times as great, extending from southern Missouri nearly to Memphis, a distance of over one hundred miles, and from one side of the Mississippi Valley to the other, and were often many feet in width. No sand is reported to have been thrown from the cracks at San Francisco except in rare cases, but at Charleston numerous craterlets such as shown in one of the accompanying illustrations were formed, from which large amounts of sand and water flowed out quietly upon the surface (Fig. 6). At New Madrid the sand and water not only came out more frequently and covered a larger area, but were ejected with violence, sometimes reaching, according to observers, to the very tree tops.

Little change of level occurred at either San Francisco or Charleston, but in the New Madrid region great areas sank and were covered by water, one of them now covered by Eeelfoot Lake being over twentyfive miles long and more than five miles wide.

The shocks in each case have had their origin in the breaking and slipping of hard rocks underneath. All rocks of the earth's crust are subjected to stresses of different kinds, such as may be produced by the weight of overlying material, by the shrinking of the earth's interior, or by other causes, and the time comes when their strength is no longer sufficient to resist them, and a break occurs, usually accompanied by a crushing of the rock along the fracture or by a slipping of one part of the rock over the other. It is this slipping or crushing which gives rise to the vibrations known as earthquakes.

In the San Francisco region this slipping is constantly going on and minor shocks have been of frequent occurrence. It was only a slightly larger slip than usual which produced the recent disastrous shake. In Charleston the slipping was mainly at one time, no preliminary shocks of importance were felt and few occurred afterwards, except during a short period immediately following the earthquake, but in the New Madrid country the quaking has continued for several hundred years at least. Both the Charleston and New Madrid earthquakes occurred in regions where the earth's crust is being overloaded—in the one instance by the sediments brought down by streams from the Appalachian Mountains and in the other by the floods of the Mississippi—and the fracturing is believed to have resulted from the readjustment of the harder rocks to the increasing load.