248 RECORD OF SCIENCE FOR 1887 AND 1888. 



cussed during- the past year by Crosby ;* and it was sLown that during 

 Silurian and Devonian time the region now upheaved as an isolated 

 mountain tract in the midst of a great continent must have been the bot- 

 tom of an ocean so broad and deep that sediments were not transported 

 to it in sufficient quantity to form deposits of any considerable volume — 

 that during a large part of the Paleozoic the region must have been 

 iu much the same condition as are today the abyssmal depths of the 

 south Atlantic and Pacific, in which dredges gather modern shells, Ter- 

 tiary shark teeth, and Cretaceous otoliths at a single sweep. 



The same author has recently discussed also the origin of the quartz- 

 ites, cherty concretions, and other siliceous aggregations found in deep- 

 sea deposits, and shown that they must result in large part from solu- 

 tion of the organic silica contained in sponge spicules, diatom shells, 

 etc., and subsequent deposition of the silica in the mineral form ; the 

 mineral silica thus disseminated among the mechanical sediments some- 

 times segregating therefrom in solid nodules or the hollow bodies called 

 geodes.^ 



VULCANISM. 



The recently renewed volcanic activity on the Hawaiian Islands led 

 to an interesting incident — i. e., a second visit to the islands by the 

 venerable Dana, for the purpose of re-examining in the light of modern 

 knowledge the phenomena studied by him half a century before. Cer- 

 tain results of the new observations werci summarized in a general 

 discussion of volcanic action, in which the principal movements are 

 classified as follows: (1) the ascending movement of the liquid rock 

 in the subterrranean conduit of the volcano from the deep-seated 

 crustal or sub-crustal region of fusion ; (2) the projection of the lava 

 aerially upward from the surface of the liquid lava in the vent, which is 

 accompanied by outflow whenever the height reached by the lavas is such 

 that thev find an outlet either over or through fissures in the walls of 

 the crater. Each of these operations (upflow and outflow) involves an 

 expulsion of material from subterranean regions ; and a usual conse- 

 quence is (3) a subsidence or down plunge of the overlying rock or of 

 portions of the cone. These movements of upthrow, ejection, and sub- 

 sidence are the most universal and strongly marked of the entire series 

 of volcanic phenomena. Connected with them, there are in addition 

 however (4) the escape of vapors from the crater, and (5) displacements 

 and the opening of fissures. | The last movements are freq uently accom- 

 panied by earth tremors, and sometimes by destructive earthquakes. 

 These observations upon the behavior of living volcanoes throw light 

 upon the history of the extinct volcanoes whose craters, lava sheets, or 

 even "necks" alone remain for examination by the geologist. 



* Proc. Bost. Soc. Nat. Hist., vol. XXiii, pp. 501-6. 



tTecbnology Quarterly, 1888, pp. 397-407; Scientific Americau Supplemeut, 1888, 

 vol. XXVI, pp. 104(;G-'fi8. 



t Am. Jour., Sci., 1887, 3il .series^ vol. xxxiii, p. 103. 



