Related Geological Conditions 47 



have been aided by their mutual repulsion resulting from 

 a highly electrified condition." 



"The crystalline particles in the mass would consist 

 of fragments of felspar, with a specific gravity ranging 

 from 2.54 to 2.75, of fragments of pyroxene with densities 

 of 3.3 to 2-5^ and of magnetite, with a density of 5.0. The 

 crystals of felspar, hypersthene, and augite were, in the 

 original pumice, of much greater size than the magnetite. 

 But the easy double cleavage in the felspar, and to a smaller 

 extent in the pyroxenes, would facilitate the reduction of 

 these minerals to finer particles than the magnetite." 



As the particles travelled outwards from the centre, 

 they would tend to fall, therefore, in the following order: — 

 (i) magnetite (the heaviest and least friable material); 

 (2) pyroxenes (next in weight and only moderately cleav- 

 able) ; (3) felspar (lighter and very cleavable) ; and (4) 

 and last, the very light and friable glass." 



"At all points therefore the dust which fell would have 

 a tendency to differ in composition from the pumice out 

 of which it was formed. Near the volcano the abundance 

 of the crystalline materials falling, and especially of the 

 magnetite and the pyroxenes, would render the dust darker 

 in color and more basic in composition; while farther away 

 the glass and felspar particles which fell, would have a 

 smaller admixture of the more basic materials. A certain 

 proportion of the glass, including the ultramicroscopical, 

 the elongated and the very thin particles would float almost 

 indefinitely and would not find any place in the masses of 

 dust collected around the volcano." 



The number of active or recently extinct volcanoes 

 throughout the world has been estimated by Judd as ap- 

 proximately 1000. But the relative thinness and plasticity 

 of the crust during palaeozoic times, must have favored a 

 much larger number, and such is consonant with the rock 

 record. Even in Mesozoic and Coenozoic times pronounced 

 activity is indicated, though the volcano-rents themselves 

 have often been obliterated. Furthermore, while the act- 

 ivity has mainly been exhibited from the cones or higher 

 points of land areas, it must frequently have been sub- 

 marine, and productive of destructive tidal waves. 



