OUR GREATEST EARTHQUAKES 85 



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 reach- 

 ing, according to observers, to the very tree tops. 



Little change of level occurred at either San Francisco or Charles- 

 ton, but in the New Madrid region great areas sank and were covered 

 by water, one of them now covered by Eeelfoot Lake being over twenty- 

 five miles long and more than five miles wide. 



Cause of the Shock's 



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 in- 

 terior, 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 pre- 

 liminary 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 earth- 

 quakes 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. 



