VULCANOLOGY AND SEISMOLOGY. 295 



Professor Dana (Am. Jour. Sci., xxxii, 247) calls attention to the way 

 In which the great erosion of the volcanic peaks in the island of Tahiti 

 has displayed the iuuer structure of this mountain and so has made 

 known what is probably the inner structure of many extinct volcanoes.* 

 The central mass of the mountain is made up not of bedded lavas, but 

 of crystalline rock eroded into deep valleys separated by "knife edged" 

 ridges and showiug no horizontal lines but rather indications of vertical 

 striation. He interprets this as being the solidified mass of lava which 

 when liquid filled the interior of the volcano, and from which, since it so- 

 lidified, the crater walls and overlying stratified beds have been washed 

 away. 



The first volume of Joseph Prestwich's Geology has been published 

 during the year (Oxford, 1886). Chapter xii is devoted to volcanoes 

 -and in it he reiterates the views in regard to the agency of water in 

 «rui)tions which he upheld in his paj^er before the Royal Society as no- 

 ticed in our summary for 1885. Chapter xiii treats of earthquakes. 

 In regard to the origin of earthquakes, while admitting other more local 

 causes, he says " I am disposed to share the view expressed by Dana, 

 that the tension and pressure by which the great oscillations and plica- 

 tions of the earth's crust have been produced havenotyet wholly ceased 

 and that this is generally the most probable cause of earthquakes." A 

 map on Mercator's projections shows the geographical distribution of 

 volcanoes and earthquakes. 



SEISMOLOGY. 



Much the most interesting occurrence of the year to Americans, and, 

 indeed, the only earthquake which has attracted any general attention, 

 was the destructive Charleston earthquake of August 31, 1886. In 

 extent of area affected and in the magnitude of the destruction which 

 ensued it surpassed anythiug that has occurred on the Atlantic coast 

 since its occupation by Europeans. 



The New Madrid earthquake of 1811, in the Mississippi Valley, was 

 probably more violent, but, owing to the sparsely populated country, 

 the material damage was much less. The first premonitions of the ca- 

 lamity were given by slight shocks felt at Charleston and Summervalle 

 on August 27 and 28, but the destructive shock occurred on the evening 

 of August 31, within a few seconds of 9.51 i>. m. By this shock nearly 

 the entire cit^^ of Charleston was ruined, almost every house being 

 more or less injured and very many rendered entirely uninhabitable, so 

 that for many days thereafter a large part of the population lived in 

 tents and other temporary shelters in the i)ublic parks. The shock was 

 felt throughout the eastern United States, as far as Boston on the 

 northeast, Toronto on the north, Wisconsin and Iowa to the northwest, 

 Missouri, Arkansas, and Louisiana on the west. It was slightly per- 

 ceptible in Cuba and the Bermuda Islands, to the south and east. Iso- 

 seismal curves have been drawn by T. C. Mendenhall (U. S. Weather 



