10 The American Geologist. January, 1905. 
far; but this is not the place for detailed results. Some- 
thing also has been done in the way of working out homo- 
geneous finite strains in rocks, so that the general nature of 
joints, faults and systems of fissures and the mechanism of 
faulting is now fairly clear. The theory of slaty cleavage 
is a subject of dispute in which I have taken part. Few col- 
leagues appear to agree with me that this cleavage is due 
to weakening of cohesion on planes of maximum slide, but I 
am not hopeless that my view will make its way to favor 
in time. 
Seismology is a vast subject by itself, but one almost 
totally lacking in theoretical foundation. Seismological ob- 
servations should afford the means of exploring the elastic 
properties of the earth throughout its interior, but the theory 
of the vibrations of a spheroid like the earth is not yet 
worked out. Meantime observations are being accumulated, 
but it can be foreseen that these will contribute little to elu- 
cidation until they include the vertical components of the 
vibrations as well as the horizontal ones. In other words, 
we must know the angle at which the wave emerges from the 
surface as well as its azimuth. The causes and conditions 
of earthquakes afford a separate topic of great interest. That 
some of them are of volcanic origin is evident ; others appear 
to be due to paroxysmal faulting, yet there is very possibly 
a common underlying cause. 
On no subjett are opinions more divergent than con- 
cerning the origin and mechanism of volcanoes. To the 
ancients they were the mouths of the river Phlegethon. To 
those who adhere to the Cartesian doctrine they are com- 
munications with the liquid interior of the earth. Most 
geologists think of them as connected with hypogeal reser- 
voirs of melted matter subsisting for indefinitely long periods 
of time. Finally it is conceivable that the lava may be ex- 
truded as soon as the melted mass has accumulated in suf- 
ficient quantity, somewhat as water may break through an 
obstructing dam after its depth reaches a certain value. The 
continual movements of the rocks show that they must be 
to some extent in a state of elastic strain, so that a given 
cubic mile of rock resists surrounding pressure in virtue 
both of its rigidity and of its compressibility. If that cubic 
