490 PROCEEDINGS OF PHILADELPHIA MEETING^ 



The paper by Professor Kemp was discussed by A. C. Lane and C. H, 

 Hitchcock. 



Following the presentation of the above paper the Society removed 

 from room 104, which was found too small, to the geological lecture- 

 room, on the same floor, where all the subsequent sessions of the meeting 

 were held. 



The next paper was read by the President, Vice-President Charles II. 

 Hitchcock assuming the chair. 



THE SHARE OF VOLCANIC DUST AND PUMICE IN MARINE DEPOSITS 



BY N. S. SHALER 



[Abstract] 



It has long been recognized that those parts of the rock ejections of volcanoes 

 which are, from their vesicular nature, in a condition to float in water, may be car- 

 ried far over the seas and in time be contributed to the deposits which are forming 

 on their floors. The object of this paper is to consider the probable amount of 

 these contril)utions and the conditions of the distribution on the ocean bottoms 

 and along the coast lines. 



The distribution of volcanoes, so far as it is known, indicates that they are inti- 

 mately related to actions going on beneath the sea floor. The facts justify us in 

 believing that these vents i)lentifally develop on the ocean bottoms, but only in a 

 limited way are formed along a stri]> of the continents next the shore, rarely if 

 ever remaining in considerable activitv after the sea has been witlidrawn for more 

 than two or three hundred miles from their sites. It is not certain that the sub- 

 marine vents which are covered by any considerable depth of water discharge ashes 

 or pumice. It is quite likely that the pressure of the overlying fluid jirevents the 

 free expansion of the interstitial steam, wliich, taking place in a limited way at 

 the surface, forms pumice, or, occurring in a more effective manner, blows the lava 

 to dust-like bits. It is, indeed, likely, though by no means certain, that, poured 

 forth in the depths of the sea, the molten rock would be retained in a rather com- 

 pact state or at most tliat the expansion of the gases would lead to tlie formation 

 of somewhat vesicular lavas. 



Limiting, as there seems reason to do, the formation of dust and jmmice to the 

 shore or shallow- water volcanoes, is there any means whereby we can obtain any 

 measure, however rude, as to the amount of the ejections of the floatable material 

 which they dischar<ie? So far T have found but one region of extended and varied 

 igneous activity where approximate estimates can be applied to the matter, that is 

 the Javanese archipela<;o, including the island of the name, Sumatra and some of 

 the neighboring lesser isles. It is a characteristic feature of the volcanoes of this 

 district that the explosions are of great intensity, little flowing lava being extruded, 

 the greater part of tlie molten rock being blown into pumice or dust. A study of 

 the eruptions which have taken place in this field during the century and a quarter, 

 endhig in the great outburst of Krakatoa in 18813, makes it appear probable that 

 the comminuted or extremely vesicular lava which has fallen upon the surface of 



