CAUSE OF EARTHQUAKES—HOBBS rare! 
areas characterized by lighter shocks; then the New Madrid earth- 
quake of a century and a half later may be considered to have accom- 
plished a similar result for the large area to the south and west, 
the valley of the lower Mississippi. Three-quarters of a century 
now elapse and the residue of the broad area, that to the south and 
east, finds relief during the earthquake of Charleston in 1886, with 
some relief also within the outlying areas where no destructive 
shocks, but only jars and tremors, were felt. 
Geological evidence is available from within the destructive area 
of the New Madrid earthquake, to show that an earthquake of 
devastating intensity and comparable in this respect with that of 
1811, visited the region at least 100 years earlier. Altogether, then, 
we have the evidence that at intervals averaging a century or 
more, this broad region of eastern North America, usually looked 
upon as especially favored by its stability, has been visited by rack- 
ing earthquakes of the first importance. It is therefore necessary 
to-day to modify in a measure the views which have been generally 
held concerning earthquake distribution. The fact that the settle- 
ment of America by Europeans came so recently, and that of the 
three great earthquakes known from the region, two belong to the 
early period of sparse settlement, pioneer scientific method, and 
imperfect historical record, largely explain the failure to assign 
proper values to these really great earthquakes. 
Epeirogenic earthquakes of De Montessus—Doctor de Montessus 
in a magistral posthumous work which came from the press in 
1924, has described a new class of earthquakes for such regions 
as lie outside the earthquake girdles. These earthquakes he ascribes 
to epeirogenic movements, up-and-down movements of neighboring 
sections of the earth’s surface layers—and hence block movements 
which are unassociated with the folding process, as are those which 
occur within the earthquake girdles.*’ In this he clearly recog- 
nized that his two earlier volumes, through laying especial stress 
upon the importance of the two earthquakes girdles, ascribed far 
too little importance to those earthquakes which occur outside. 
It is easy to account for the earthquakes of the lower Missis- 
sippi Valley through the gradually accumulating load over the delta 
region and the lower flood-plain of this great river. It has been 
estimated upon good authority that 513 million tons of suspended 
matter are carried out each year to tidewater in Louisiana, and 
this takes no account of the vast load that is laid down within 
the broad area of the flood-plain in the States of Arkansas, Mis- 
souri, Tennessee, Mississippi, and Louisiana. 
27, de Montessus de Ballore, La Géologie Séismologique, les Tremblements de Terre, 
Colin, Paris, 1924, Ch. I. 
