408 



relief of pressure acts in the same way as increase of temperature 

 so far as crystallization in igneous magmas is concerned. Again, 

 corrosion may be due to a change in the composition of the magma 

 caused by the separation of minerals of a different composition from 

 the one under consideration, or by the addition of other material to 

 the liquid mass. When a period of crystal-growth follows a period 

 of corrosion the new material may have a zonal structure which 

 stands in relation to the corroded outlines. 



Crystals formed under plutonic conditions may be corroded and 

 even destroyed under volcanic conditions. This important point is well 

 illustrated by the minerals, hornblende and biotite. These minerals 

 have never yet been synthetically produced by pure igneous fusion 

 under ordinary pressure. So far as igneous magmas are concerned 

 they appear to be in general of plutonic origin/ 1 ) When they occur 

 in volcanic rocks they are frequently surrounded by a black border 

 composed of magnetite and augite. In many cases the form only of 

 the original mineral is preserved and we may, therefore, safely 

 conclude that in many other cases all traces of original hornblende 

 and biotite have disappeared. 



The mechanical actions accompanying the eruption of igneous 

 material often produce important effects on the previously formed 

 crystals, breaking them and fracturing them in various ways. 



So far we have been referring to actions which may take place 

 before the actual appearance of the molten matter at the surface. 

 After eruption the lava flows over the surface ; the ultimate form of 

 the rock-mass depending on the fluidity of the lava and the con- 

 figuration of the district. Any want of homogeneity in the original 

 magma will produce banded or linear structures in the consolidated 

 mass. The material which was molten at the time of eruption may 

 contribute somewhat to the enlargement of the pre-existing crystals, 

 but the bulk of it consolidates to form the ground-mass of the 

 rock. The ultimate character of the ground-mass depends on the 

 extent to which crystallization is carried on after eruption. Every 

 gradation may be observed between a rock with a vitreous ground- 

 mass in which only crystallites or spherulites have been developed, 

 to one in which complete crystallization has taken place. The effect 

 of progressive crystallization on the composition of the residual glass 

 has been so frequently referred to that nothing further need be said 

 on this subject. As soon as final solidification has taken place the 

 rock is formed and liable to be modified by the agencies which we 

 shall refer to in the next chapter. 



The molten mass is frequently charged with water and other 

 volatile substances at the time of the eruption. In the case of 

 a paroxysmal eruption, such as that of Krakatoa, the water is present 



(1) LAGOEIO mentions the occurrence of hornblende -needles (5 mm. x - 5 to 1 mm.) in the 

 pores of a leucite-tephrite from Vesuvius (lava of 1822). The needles lie free in the pores and 

 do not occur in the rock. T.M.M., Neue Folge, Band VIII., p. 523. 



