32 TEANSACTIONS OF THE [OOT. 18^ 



feet above the water, is a bank of oyster shells, four feet in thick- 

 ness ; this descends toward the south, and disappears beneath 

 the surface of San Francisco Bay. Paget Sound, with its many 

 branches, is only the submerged valley of a great river which ran 

 out to sea through the Straits of Fuca when the coast was much 

 higher than now ; but the shores are terraced to the height of 

 1,600 feet above the present water-level ; showing that, in recent 

 times, they have been much lower than now. Similar facts with 

 these have been reported from the shores of all the continents, 

 and the islands afford more striking examples of the changes of 

 level ; the Windward Islands are only the summits of a lofty 

 mountain chain which was once all above the sea-level, as is 

 shown by the community of species in animals and plants. The 

 Islands of the South Pacific are, also, the summits of mountains 

 which have been gradually submerged, as has been shown by Dana 

 and Darwin. Coral reefs which are formed only within 150 feet 

 of the surface now extend down in continuous walls, 2,000 feet 

 below the water ; the growth of the coral having kept pace with 

 the gradual subsidence. Elisee Eeclus, in La Terre, and Profes- 

 sor Prestwich, in his Geology, give maps, showing the fluctuations 

 of level now in progress along coast lines ; and whoever will ex- 

 amine these maps will find it difficult to reconcile these oscilla- 

 tions of the land with a globe solid to its centre, or even with a 

 thick crust. But the changes of level now taking place proceed 

 so slowly that the record of one hundred and fifty years, during 

 which geological observations have been made, or even that of the 

 long period covered by human history, is insignificant, compared 

 with that of the geological ages. Indeed, historical geology is, for 

 the most part, but a transcript from the monuments left by suc- 

 cessive and local subsidences of the land, influxes of the sea, and 

 the deposition of strata containing relics of the marine and terres- 

 trial life of the epochs in which these inundations occurred. 

 Scarce any portion of any continent is without traces of the pres- 

 ence of the sea, and, while some of these submergences were, 

 doubtless, caused by great tides which ebbed and flowed from one 

 hemisphere into the other, in the manner suggested by Adhemar, 

 it can be easily shown that most of them were occasioned by local 

 subsidences of the land. 



All these lines of evidences, furnished by earthquakes, moun- 

 tain chains, volcanoes and terrestrial oscillations, converge to 

 one point, and, in combination, go far to prove that the earth^s 

 crust is relatively thin, and that its interior is fluid or viscous. 



A priori considerations confirm this conclusion. If, as all 

 believe, the earth was once a globe of molten matter which has 

 cooled by radiation from the exterior, it seems impossible that 

 the first formed crust could have sunk to the centre, and there 



