96 
IDDINGS. 
The presence of vapors as the molten magmas approach 
the surface inflates them and lowers the specific gravity. 
His hypothesis hinges on the idea that “ wherever the tem¬ 
perature of the liquid mass begins to be less than that 
necessary to retain the feldspar in fusion, there the feldspar 
will commence to solidify, or will slowly stiffen in the midst 
of the fluid material made up of the other ingredients. In 
this state the vapors ascending in the conduit will urge 
upward the feldspar much less freely than the more liquid 
part of the lava.” 
He adds “ that it does not necessarily follow that the lavas 
of fissure eruptions have, in all instances, been thus de¬ 
rived,” for they may proceed frbm portions of the liquid 
lavas which have not experienced this separating process, 
which only takes place where the vapors have a chance to 
ascend and escape. 
He questions whether the mineral, augite, exists in a 
molten state as a fixed compound or in some different com¬ 
bination, and considers it possible that “ the magnesia of 
the lava, or a large part of it, is engaged in combination 
and used up, as we may say, deep below,” in the formation 
of olivine. Whether it be wholly or partially used up 
would probably depend on “ the proportional amount of 
silica present and the temperature, the rest, at a higher 
elevation in the lava conduit, going to the formation of 
augite or some allied compound.” 
As to the production of syenite he remarks: “ The feld- 
spathic center is enclosed within a thick covering of rocks, 
and will therefore cool slowly; and, though generally form¬ 
ing only disseminated crystals of feldspar in an earthy 
base, the cooling is sometimes sufficiently gradual to allow 
of the whole crystallizing, and in this case the texture 
throughout is crystalline and the rock much resembles a 
granite. Under the same circumstances (or even a less 
gradual cooling) the elements of augite present will crystal¬ 
lize as hornblende.” * * * “The same remarks will 
also apply to mica,” while “ the excess of quartz not in 
combination ” will behave like the feldspar. 
