CONCLUSIONS 803 



CAUSE OF FLUIDITY 



The preceding descriptions of the plateau basalts, showing that one of 

 their main chemical characteristics is the high percentage of iron oxides, 

 and especially of ferrous oxide, furnish an explanation of their great 

 fluidity at the time of extrusion. It is a matter of common observation 

 that basalts generally are fusible at a lower temperature, and are more 

 fluid when molten, than are more feldspathic or more silicic rocks. It is 

 also well known that ferrous silicates are more readily fusible than are 

 magnesium or calcium silicates. The experience of iron and steel workers 

 and smelters bears testimony to the lower fusibility and greater fluidity 

 of slags containing considerable iron. This is the opinion expressed to 

 me orally by Mr. J. A. Aupperle, of the American Rolling Mill Company, 

 and Dr. W. G. Fearnsides, of Sheffield. In a recent letter Mr. Aupperle 

 communicates the conclusions arrived at in the works of the American 

 Boiling Mill Company ; these are so pertinent that part of his letter may 

 be cited : 



"From our observations we firmly believe tbat the higher the iron oxides in 

 slags the more fluid these slags will be, as we have noticed that the slag from 

 Armco Ingot Iron, which contains the highest iron oxide, is extremely fluid. 

 When high iron oxide slag ... is poured on the ground in large masses it 

 freezes immediately to a very compact mass, while slags containing lower iron 

 oxide form porous masses and are not so homogeneous. The slags in the slag 

 pockets of our open-hearth furnaces are also high in iron oxide, and they are 

 likewise very fluid." 



The same general relation is expressed in papers on iron slags by J. H. 

 Whiteley. 45 Inasmuch as the fissure eruptions which furnished the pla- 

 teau basalts show slight explosive activity, it is to be inferred that the 

 magma contained comparatively little gas, so that the effect of this class 

 of components in lowering the fusing point or increasing the fluidity 

 should be less than in ordinary basalts of the explosive cone type. We 

 seem, therefore, to be justified in ascribing the peculiar physical condition 

 of these basalts during their extrusion chiefly to their high iron content. 



45 J. H. Whiteley and A. F. Hallimond : Jour. Iron and Steel Inst, vol. 99, 1919, p. 

 199 ; and J. H. Whiteley, op. cit, vol. 101. 1920, p. 308. 



