ROCKS 3D 
Frequently, the crystallites, microlites, crystals, and spherulites contained 
in a glass appear arranged in lines or in bands ; very often, too, the glass 
shows a ribboned or striped appearance—darker and lighter coloured 
layers rudely alternating (see Plates VIII. 1,2; XIII.2). This is known 
as Fluxion or Fluzdal structure, and is obviously due to the differential 
movement of the rock while it was still in a mobile condition. 
All molten masses contain water and various vafours and gases, which 
are given off in dense clouds from a lava at the time of its eruption. 
When the lava is very liquid the steam readily escapes ; but as the mass 
on cooling becomes more viscous the vapours are less easily got rid of. 
They segregate and expand, pushing the plastic rock aside and thus 
forming spherical cavities. In this way the upper portion of a lava is 
often rendered more or less vesicular. As the lava flows on its way 
the spherical cavities become flattened and drawn out in the direction of 
movement. The vesicles vary in size from mere pores up to cavities, 
measuring more than one foot across; but cavities of such a size occur 
only sporadically. In the case of vitreous rocks which have flowed out 
ina highly liquid condition, the vesicles are rarely large. Usually they 
are so small and so very abundant that they may occupy fully as much 
space as the solid portion which contains them. Vitreous rock of this 
kind has a spongy, froth-like appearance, and is known as Pumice. The 
vesicles formed in very viscous lavas are usually larger and not so 
abundant. 
Hemicrystalline Rocks. 7Zheir General Character.—These rocks are 
composed chiefly of crystalline ingredients, with a larger or smaller 
proportion of non-differentiated matter. Typically, a hemicrystalline rock 
contains the following constituents :—(@) Groundmass, an aggregate of 
microlites and small crystals or crystalline granules, with which 
some amount of glass (not infrequently devitrified) may or may not be 
associated ; (0) Phenocrysts, the term applied to the larger crystals 
disseminated through the groundmass, 
Most hemicrystalline rocks have consolidated at or near the surface 
of the earth. While they were still in a molten condition, however, and at 
some considerable depth in the crust, cooling had already commenced, 
and certain minerals had crystallised out. Such minerals, therefore, 
eing free to develop, often attained a relatively large size and a more 
or less perfect crystalline form. Not infrequently, however, they show 
corroded outlines, as if they had been partially dissolved. This is 
supposed to have been caused by the action of the still fluid portion of 
the magma—rendered more acid as it would be after the phenocrysts 
had separated out. Probably the process of resorption was aided also 
by changes of pressure and temperature as the molten rock rose towards 
the surface. Not only are the phenocrysts frequently corroded, but they 
have often been broken during movements of the magma. Thus, when a 
molten mass eventually reached the surface, it already contained many 
disseminated solid particles—the phenocrysts. No sooner did the lava 
begin to flow than cooling proceeded so rapidly that large and 
approximately perfect crystals could no longer be formed—the numerous 


