BY W. N. BENSON. 165 



3. Albite-magnetite-quartz-magma, with abundant smooth- 

 walled vesicles, and evidence of the presence of magmatic water. 

 Still further diminished viscosity, brecciation practically absent, 

 and flow-structures more obvious. 



4. Quartz-albite-magma, with abundant amygdules filled with 

 silica, and evidence of the former presence of much magmatic 

 water. No sign of brecciation, but every indication of consider- 

 able fluidity. 



5. Quartz, chalcedony, and haematite, deposited from aqueous 

 solution. 



The knowledge of the relation between magma-viscosity and 

 chemical composition is at present very imperfect(40), particularly 

 in regard to the quantitative effect of fluxes, such as water; 

 nevertheless, the sequence given above seems to accord with what 

 might have been anticipated. 



So far, only those jaspers that are immediately adjacent to the 

 spilites or keratophyres can be said to have been derived from 

 this source, and such jaspers are as yet known in the Bowling 

 Alley Series only. The mode and period of origin of the far 

 more abundant jaspers of the Woolomin Series are not yet known. 

 They show many of the features common to the other jaspers, 

 though they are more uniform in character, and less vein-like. 

 The writer concurs with Professor David's present opinion, that 

 they are mainly of secondary origin, alteration-products or meta- 

 somatic replacements of country-rock. They can hardly be 

 merely ferruginous, abyssal oozes, as formerly suggested. 



The formation of ferruginous jaspers and iron-ores by solutions 

 derived from spilitic magmas is not without analogy. The same 

 mode of origin has been claimed for much of the Lake Superior 

 iron-ore(37), as also for the ores of the Rhenish Schiefergebirge 

 in Germany(39) and elsewhere. In these cases, however, the 

 iron-bearing solutions are believed ^to have escaped from basic 

 lava-flows, and not after extreme differentiation. 



Difficulties arise when one endeavours to determine the con- 

 ditions under which the series of eruptions took place, which 

 produced the rocks described. The spilite-pillows must have 

 invaded sediments that were still watery, and capable of fluid 



