152 Scientific Proceedings, Royal Dublin Society. 



lithic plates, and small crystals may be seen scattered throughout 

 the magma, and often enclose crystals of orthoolase. In the more 

 highly crystalline pumices and lavas this mineral occurs generally 

 as well- formed crystals. Although it is not very uniform in its 

 occurrence, I am disposed to regard it rather as pre-eruptive in 

 formation, or, at any rate, in part. 



Magnetite is another mineral that oannot be obtained by 

 simple fusion, but requires solution in a fused medium, from 

 which it separates during cooling within a great range of tem- 

 perature, 1 provided the formation of other minerals renders the 

 magma supersaturated, from time to time, with this oxide, so that 

 various crops of crystals may result, forming so many periods of 

 consolidation. This is the only way we can explain its formation 

 as with quartz, leucite, &c. Scheerer pointed out long ago 

 the granite-forming minerals separated inversely to their fusion- 

 points. 



Pyroxene, as well known, is a common product in furnace 

 slags, and is easily obtained by simple fusion of its elements with 

 a very short recuit. Messrs. Fouque and M. Levy found it to be 

 produced in a microlithic condition after a few moments' recuit, 

 and prolonging this a little, fine crystals, such as are met with in 

 volcanic rocks, were obtained. Such a fact convinces us of the 

 extreme rapidity with which basic pumices, at any rate, must 

 have passed from the fluid to the solid condition, as in many of 

 the Italian basic volcanoes the first products of some of their 

 explosive eruptions were practically without even microliths of 

 pyroxene, striking examples of which are to be met with in the 

 deposits of Phase III., period 1, and Phase VI., periods 1 and 3, 

 of Monte Somma. The above-mentioned authors found the 

 limit of temperature rather wide in which this mineral crys- 

 tallized, which accounts for its inclusion in others that separate 

 at rather higher temperatures. The pyroxenic glass seems to be 

 the principal medium in which the other silicates and oxides are 

 dissolved in basic rocks, whereas an acid felspathic glass seems 

 to perform the same function in acid ones. 



We may regard the magma from which results an igneous 

 rock as a variable mixture of acids and bases, as pointed out by 



1 Bull. Soc. G£ol. 2 e serie, torn. iv. page 47S, 



