216 
THE GEOLOGIST. 
Appalachian chain of Mountains in North America, communi- 
cated by Professors W. B. and H. D. Rogers, and intended to 
illustrate the law of structure prevailing throughout that chain. 
" On the Phenomena and Theory of Earthquakes, and the ex- 
planation they afford of certain facts in Geological Dynamics by 
H. D. and W. B. Rogers. 
The authors first proceed to examine the phenomena attending 
earthquakes commencing with the following propositions: — 1. 
Earthquakes consist essentially of a wave-like motion of the whole 
solid ground. 2. The earthquake undulation is not simultaneous 
throughout the whole area agitated, but is progressive, and propa- 
gated with enormous velocity. 3. The undulation is transmitted 
sometimes in the manner of an elongated, curved or nearly straight 
belt, moving parallel to itself, and sometimes in the manner of 
a dilating elliptic or circular zone. The author then described 
earthquake phenomena as divisible into such as are of invariable 
occurrence, and therefore characteristic, and such as are only 
occasionally witnessed. The characteristic phenomena consists as 
announced by the Rev. John Mitchell, of Cambridge, in the 
Phil. Trans, for 1760, of a peculiarly rapid undulation, or wave- 
like motion of the ground, and a sharp vibratory jar, or tremor ; 
the undulating generally extending further from the source of 
the earthquake than the tremor. The occasional phenomena, 
observable only when the earthquake is violent, are a deep rum- 
bling and grating noise, an alternate opening and closing of 
parallel fissures, and the escape of steam and sulphureous and 
other vapour, and hot water, from those fissures. In confir- 
mation of these observation, the author cites the earthquake of 
Conception in 1835, described by Captain Eitzroy, and that 
which visited the island of Hayti, in May 1812. As examples 
of the second proposition, they mentioned the earthquake of 
Lisbon, and two others which have occured during the present 
year. The first of these took place on the 4th of January, 
and was felt along the valleys of the Mississippi, from the mili- 
