Shulcr.J 252 [March 7. 
we have no liistoric evidence of great shocks. As I am not 
aware tliat any other observer has undertaken a study of 
this nature, I liave been compelled to devise my own means of 
inquiry. The basis of experience is not sufficiently great for 
assured conclusions, but I venture to hope that others who are 
better fitted for the task may undertake to extend these considera- 
tions. 
The influence of an earthquake shock on topographic features 
has often been desci-ibed. Where the shock is of moderate 
energy, even though it prove destructive to weak architecture, 
there is generally little effect on the earth's surface which can be 
observed after any consideiable interval of time. The simplest 
and most general topographic effect exercised by earthquakes of 
moderate intensity is found in the action they produce on 
extensive alluvial plains or other deposits in which a considerable 
amount of chemical change takes place in porous strata lying near 
the surface. Thus the earthquake of Charleston, S. C, though 
a shock of no great violence, exhibited in a sufficient manner the 
peculiar influences of such convulsions on the above-described 
kinds of rocks. In such alluvial or other porous superficial 
deposit, the rainwater penetrating through the humus layer finds 
its way downward to a considerable distance. It exercises a 
certain solvent action tending in ;i limited way to create cavities 
beneath this surface and to increase the storage of water within 
a few feet of the air. The penetration of this water and the 
processes of metamorphism due to other causes generate a 
considerable amount of gas, carbon dioxide and other gases, of 
decomposition, which seek to escape but not with sufficient enei'gy 
to overcome the resistance of the overlying matter. Any con- 
siderable earthquake tremor of the area releases more or less of 
this gas which impels a portion of the water along joint planes 
and through other lines of weakness to the surface, where it breaks 
forth from crevices situated on such lines of fractures. Thus in 
the Charleston earthquake, as before remarked, though one of 
relatively slight energy, a large part of the region affected by 
the movement poured forth its subterranean waters from thou- 
sands of openings. Over a large part of the region near 
Somerville and at other points in the shaken district, these pits 
were extremely numerous and they will long remain as distinct 
indications of the disturbance. Some of them are sufficiently 
