262 Russell — Massive- Solid Volcanic Eruptions. 



that condition have any characteristics in their chemical com- 

 position which will serve to differentiate them from other lavas % 

 The reply, so far as it can at present be formulated, must evi- 

 dently be based on the analyses of the rocks known to have 

 been extruded in a massive-solid condition and a comparison of 

 them with the composition of lavas not known to have been 

 erupted in that manner. 



In the above table the available analyses of the examples 

 of massive-solid eruptions described on the preceding pages, 

 have been assembled, together with an analysis of a characteris- 

 tic phonolite (JSTo. 4) to represent the rock of the Peak of Fer- 

 nando de Noronha, and also an analysis of basalt (No. 5) which 

 is known to have been erupted in a highly fluid condition. 



To the examples of massive-solid eruptions cited in the 

 table, should be added an analysis of the augite-andesite of 

 the crater near Pauline Lake, Oregon, described on a previous 

 page, but this data is not available. 



As indicated by laboratory experiments made on the fusi- 

 bility of shales and clays* similar in chemical composition to 

 the rocks enumerated above, the relative fusing points of such 

 rocks may be roughly or qualitatively expressed by the ratio 

 of the silica and alumina to the iron oxides, magnesia, lime, 

 soda and potash, or the "fluxes " present in them. The rule 

 being that for mixtures of the general nature that lavas pre- 

 sent, the higher the ratio of the silica and alumina to the fluxes 

 present, the greater the degree of heat necessary to cause 

 fusion, under laboratory conditions. On arranging the analyses 

 given in the above table, so as to indicate the v atio of acid to 

 basic substances given, we have the following :f 



Table Showing Relative Fusibility. 



Si0 2 + Al 2 3 . 



1. Rhyoliteof Mono Craters 87*90$ 



2. Andesite of Pele 78*71 



3. Phonolite 78*05 



4. Andesite of Bogosloff... 75*13 



5. Basalt of Cinder Buttes. 65*09 



As indicated by the ratios given in the third column of 



* H. Kies, Clays and Shales of Michigan, in Geological Survey of Michi- 

 gan, vol. viii, Part I, 1900. 



f This table is only approximately correct since no account is taken of the 

 water present ; and in the case of the numbers 2 and 5, no account is taken of 

 the several minor constituents given in the analyses as originally reported. 



% According to experiments by C. Barus, quoted in J. D. Dana's Manual 

 of Geology, 4th ed. 1895, p. 273. 



'Fluxes. " 



Eatio of 



Si0 2 + 



A1 2 3 to 



"Fluxes." 



Approximate 

 temperature 

 of fusion 4 



9*88$ 



8*8 



3100°F. 



19*88 



3*9 ) 





21-40 



3-6 [ 



2520 



21*89 



3*4 ) 





29*81 



2*2 J 



2250 



