372 The American Geologist. December, 1893 
while joints of even moderate regularity are not satisfactorily 
explained by torsion if occurring in incoherenl deposits or in 
rocks of irregular structure. 
Jointing is the most universal of rock structures, and with 
respect to the universality of the agents, there is little to 
choose hetween the two theories under discussion. The ob- 
servations of professor Niles* and others confirm the belief 
which, no doubt, exists independently of specific facts in the 
minds of most geologists, that the relatively superficial strata 
of the earth's crust are very generally, if not almost every- 
where, in a state of strain — compressive, tensile, or torsional ; 
and it will be readily conceded that probably all parts of the 
earth's crust have been traversed by severe earthquakes. 
It has occurred to me, however, during the past year, while 
preparing a set of lecture notes on joint structure, that some 
of the perplexities of the subject would be removed by com- 
bining the torsion and vibration theories. It is a familiar 
fact that when a body is in a state of strain a shock or jar 
will often precipitate the yielding or collapse. If the strain 
is near the breaking intensity, a very slight shock is sufficient : 
and in such a case the directions of the fractures will be de- 
termined wholly by the pre-existing strain and not in any sen- 
sible degree by the vibrations. The function of the latter is 
simply to bring the strain instantly to the breaking intensity 
and thus insure straight and regular fractures along the pre- 
determined lines. Torsion yields simultaneously two rectan- 
gular systems of fractures; while the earthquake vibrations 
produce plane and regular fractures in almost all kinds of 
rocks, [n other words, if eo-operating, torsion would natually 
determine the directions of the fractures, and the vibrations 
the time and modi- of breaking. Simply conceive, then, that, 
as must often happen, while a torsional or plicating strain is 
being slowly developed in the rocks and is yet considerably 
below the breaking intensity, they are traversed by an earth- 
quake of moderate severity. Although the shock is insuffi- 
cient in itself to break the rocks, it will precipitate the break- 
ing that is imminent through bending. Simple plication 
would then yield one system of regular joints, and torsion 
two systems. The main point is that the torsional strain. 
*Proc. Boston Society of Natural History, xvm, 272. 
