XVI. PRESIDENT S ADDRESS. 



SO cooled with comparative rapidity. It is quite possible that 

 the differences observed in lavas as to the amount of crystalliza- 

 tion to which they have attained may be connected with 

 differences in the amount of this annealing process to which 

 they have been subjected. If the mass is erupted in a condition 

 of tolerably perfect fluidity it will more easily pass into the 

 glassy state than if by slow cooling in the interior of the earth 

 it has been elaborated and prepared to develop crystals on 

 eruption and consequent cooling. Thus on the one hand we 

 have the almost unique lavas of Mauna Loa, in the Sandwich 

 Islands, which solidify almost constantly as glass, and on the 

 other the lavas of Etna, which contain very little residual glass, 

 except such as is shut into the larger crystals. 



It is also probable that somewhat similar differences have 

 determined, in the case of dykes, whether there shall be a 

 boundary band in a glassy state, or whether the whole shall be 

 crystallized. In the first case the injected mass must have been 

 quite fluid, while in the second we may either suppose that it 

 had already cooled to a viscous state, and had been maintained 

 in tliat state for some length of time, or else that the whole 

 mass of strata into which the eruption took place was at such a 

 high temperature, or was of such bad conducting character, that 

 the mere time necessary for the cooling allowed a sufficiently 

 prolonged viscous stage for crystals to form from the beginning 

 of the actual solidification. 



The results which our authors have arrived at show that 

 taking a fused mass, of composition similar to that of one of 

 the basic rocks, we can, by making two stages of the cooling 

 process, reproduce even the most minute details of the natural 

 products. The examination of these show that the first stage 

 of the consolidation has generally resulted in the formation of 

 larger crystals than the second. Some minerals are found 

 only as products of the first stage, such as olivine, which is 

 never met with in actually microscopic grains. The leucite, 

 again, has always separated before the felspar and augite, 

 which often accompany it, though in this case the fact that 



