PROCEEDINGS OF GEOLOGICAL SOCIETIES. 



4G7 



Lecturk III. {Nod. 20.) — Volcanos inoiintaiiis of cjeclion atid ncciirniilalioii, Idil: nut 

 of elevation. Researches on this siil)jeet of Mr. Sei-ojx:, Sir Chailes Lyell, and M. 11. de 

 Saiissure. Peeuliar conieal form and structure of volcanic luountaiiis, their dis])o>iliou 

 in groups or lines connected with fissures in the earth's superficial crust, and Geo;£rap!ii- 

 cal Distribution. May he slnwly or rapidly produced, but, when formed on the land, 

 are always of long duralioii. Have prtjijably existed at all known g(;o]ogieal ])eriods, and 

 some which were active in the later Tertiary or Cahiozo'u; eras arc active now (as l^tna), 

 while others arc extinct (as the voleaiios of Central iM-ance and Asia Min(ji-j. One vol- 

 canic cone may envelope and bury olhcrs in its ejections, or one chain of volcanic vents 

 may overwhelm anotiier under its lavas and lull's, and so ])reserve them as a part of it- 

 self, as observed in Madeira by Sir Charles Lycll and M. ilartuiig; but as voh;auic 

 cones cannot in any otiiei- sense become subterranean, nor long i'cinain beiu'ath the sea, 

 there are not any Fus,sil I'olcitiios. Of those, thererore, wliieli were aelivc in the .Me>()- 

 zoic and Pahcozoic eras of geology, certain products only remain. Siieh prodncl ^ ;il)iin- 

 daut in Britain, as exemplitied in the micklle series of Cambrian Slates, and the W e^iern 

 Islands of Scotland. ]n what manner volcanos are related to the elevati(ju of the land 

 and of other mountains, and to the production of the actual sui'facc of the globe. Pro- 

 bably only the features of that surface produced by the direct action of the internal 

 forces, as recently indicated b)- Mr. Jukes, ihioi-mous mass of matei-ials transferred by 

 them from below to tlie surface. 



Lectukk IV. {Dec. 8.) — \ Oleanos not the cause of earth([ual<es iu)r eai'th(pialx(^s of 

 volcanos, as eoniuu>nl\' supposed ; but both the" manifestations of a common force under 

 dill'crent conditions," or the partial ell'ects of a common cause, or of a common series of 

 causes, originating in the internal heat of the globe. Particular history of some of the 

 most remarkable eartluiuakes. — Those; of Lisljon, November 1st, 1755; Calabria and 

 Sicily, February and Mai eli, 17^3: Kiobamba, in Peru, February 4th, 1797, the greatest 

 earthquake whose ell'ects have be(;n observed; Chile, l^'ebruary 2{)th, 1835, and concomi- 

 tant volcanic phenonuMia ; Souiliei'u Italy, IJeceinber lOth, 1857. Succession of earth- 

 quake phenomena when taking place on laud or under the ocean. The ground and all 

 objects resting upon it suddenly moved backwards and forwards by an alternate horizon- 

 tal motion, aeeomj)anied by a vertical or u[)\vard and downward motion. Duration of 

 the shock. A continuous violent ti'enior often felt in addition. Great sea-wave attend- 

 ing earthquakes. Sounds which accompany them when subterranean fractures occur. 

 Earthquakes do not occasion permanent elevation or depression of the land or sea-bed 

 (for reasons to be explained in the next lecture), contrary to what is often affirmed by 

 geologists, though such changes of level may take ])lace at the same time. Occur over 

 all parts of the earth's surface, but certain areas of land and sea more subject to them 

 than others. Geographical distribution of these areas, or Sdsntic regions, and its rela- 

 tion to that of volcanos. In what manner the occurrence of earthquakes and the erup- 

 tions of volcanos are related. Alleged and possible influence of the unecpuil attraction of 

 the moon, and of the varying pressure of the atmosphere on the occurrence of earth- 

 quakes. Certain districts of Great Britain subject to slight earthquake-shocks at the 

 present time. 



Lecture V. {Dec. 10.) — The philosophy of earthquakes constitutes the new depart- 

 ment of science termed Seismology. Analysis of their phenomena. The shock, or earth- 

 quake-wave, a true roll, or contimied undulation of the solid crust of the earth. Earth- 

 quake-motion shown to be undulatory, or ivave-like, by Michell, about a century ago; 

 but erroneously assimilated by him to that of ordinary liquid superficial' waves, such as 

 tliose of the sea, which it only apparently resembles. The Dynamics of Earthquakes first 

 explained, and the true nature of their motion, as being that of a Wave of Elastic (\)m- 

 prcssion, demonstrated, by Mr. Kobert Mallet, in 181(). ^Vaves of elastic compression 

 may be communicated to, or oiiginate within, the substance of matter of every kind, 

 aeriform, licpiid, and solid ; as the atmosphere, the waters, and the earth; becoming sen- 

 sible as sound, in all three nu'diums, and also, in the case of the earth-wave, as Seismic 

 or eartiiquake i)henomena. These waves consist of an alternate condensation and rare- 

 faction of the medium; the ])arti(les of lluid or solid alteriuitely approaching and receding 

 from each other, in continuous succession, ami in all directions from the central or 

 focal point where the impulse takes place which gives origin to the wave. An earlh- 

 quake the transit or passage of such a wave, or of a succession of such waves, through 



