200 The American Geologist. March, 1891 
error of less than two miles ; and the depth of the more southern focus, 
though less definitely known, is believed to have been about eight miles. 
Both t hr course of the isoseismal curves and the absence of any apparent 
changes in the slopes and currents of the streams in the district seem to 
show that there was no extended faulting movement, shearing the rocky 
strata along an extent of many miles. 
The tremors from the most violent shock, which occurred at 9 h. 51 
m. 6 s., very closely, in the evening of August 31st, were propagated at 
a speed of about three and a quarter miles per second, or nearly 200 
miles per minute, to so distant points as Boston, Milwaukee, La Crosse, 
Keokuk, eastern Arkansas, and New Orleans. They were also felt in 
€uba and in Bermuda, the distance to the latter island being almost 
exactly 1,000 miles. The area within which the motion was sufficient 
to be noticed, including its oceanic portion, was approximately 2,500,000 
square miles. With instrumental observation, such as is obtained in 
Japan by self-recording seismographs, the disturbance would doubtless 
have been detected over a much larger area. Because of the general 
use of the standard time system, and the harmony of the observations 
giving the time of this earthquake in different parts of the country, the 
author regards his results for the speed of transmission of the shock as 
far more accurate than all previous determinations for other earth- 
quakes. And it is noteworthy that this velocity agrees with that which 
theory indicates for elastic waves in an indefinitely extended solid mass 
of siliceous material, as captain Button thinks the earth to be to the 
•depth of at least a hundred miles from its surface. 
A vast amount of detailed information was gathered, mainly through 
assistance by Mr. Earle Sloan of Charleston and by Messrs. W J McGee 
and Everett Hayden of the U. S. Geological Survey ; but, with most care- 
ful study, the question how the earthquake was caused completely baffled 
the long continued and painstaking efforts of the author to obtain any 
answer. It seems therefore wellnigh certain that the premises on which 
his investigations proceeded were somewhere unsound. May not the 
solid mass of the crust of the earth, while having a physical constitution 
competent for the transmission of the earthquake shock as observed, be 
yet much thinner than captain Dutton supposes '? If the earth's crust 
rests, at a depth varying perhaps from twenty to thirty or forty miles 
beneath its surface, on a heavier viscous or liquid interior of molten rock, 
or of rock under combined conditions of intense heat and pressure which 
render it freely plastic, may not strains resulting from slow cooling and 
shrinking of the interior, and from the transportation of material on the 
surface by streams, produce faults in the solid crust at such depths and 
of so limited length as to be manifested by epicentral tracts like those of 
the Charleston earthquake? This view seems to accord well with the 
phenomena and distribution of volcanoes, with the great movements of 
folding and faulting by which mountain ranges have been formed, and 
■with the epirogenie uplifts and depressions which have affected broad 
areas, as the whole or large parts of continents. 
