chap, in 
STRUCTURES OF LA VAS 
21 
magma iu its molten condition. During devitritication, however, other fel- 
spars of a later period of generation made their appearance, but they are 
generally distinguishable from their predecessors. I’robably most liasic and 
intermediate rocks, when poured orit at the surface as lavas, were no longer 
mere vitreous material, but had already advanced to various stages of progress 
towards a stony condition. These stages are still to some extent traceable 
ioy the aid of the microscope. 
Jlicrolites of the component minerals are first developed, which, if the 
process of aggi’egation is not arrested, build up more or less perfect crystals 
or crystalline grains of the minerals. Eventually the glass may be so com- 
pletely devitrified by the development of its coustitutent minerals as to be 
wholly used up, the rock then becoming entirely crystalline, or to survive 
only in scanty interstitial spaces. In the family of the basalts and dolerites 
the gradual transition from a true glass into a holocrystalliue compound 
be followed with admirable clearness. The component minerals have 
sometimes crystallized in their own distinct crystallographic forms (idio- 
morphic) ; iu other cases, though thoroughly crystalline, they have assumed 
externally different irregular shapes, fitting into each other without their 
proper geometric boundaries (allotriomorphic). 
-b specially characteristic featiire of many basic rocks is the presence of 
"’hat is termed an Ophitie structure (Fig. 7). Thus the component crystals 
pyroxene occur as large plates separated and penetrated by small needles 
and crystals of felspar. The portions of pyroxene, divided by the enclosed 
^olspar, are seen under the microscope to be- in optical continuity, and to have 
crystallized round the already formed felspar. This structure is never found 
in metamorphic crystalline rocks. It has been reproduced artificially from 
fusion by Messrs. Foiupui and Michel Levy. 
The name Variolitic is applied to another structure of basic rocks 
ifig. 8), in which, especially towards the margin of eruptive masses, abund- 
ant spheroidal a<Tgregates have been developed from the size of a millet-seed 
cn that of a walnut, imbedded in a fine-grained or compact greenish matrix 
into which the kernels seem to shade off. These kernels consist of silicates 
arranged either radially or in concentric zones. 
3, Flow-structure is an arrangement of the crystals, vesicles, spheru- 
lites, or devitrification-streaks in bands or lines, which sweep round any 
enclosed object, such as a porphyritic crystal or detached spherulite, and 
^'epresent the curving fiow of a mobile or viscous mass. Admirable examples 
ef tills structure may often be observed iu old lavas, as well as in dykes and 
®^lls, the streaky lines of flow being marked as distinctly as the lines of foam 
tlint curve round the boulders projecting from the surface of a mountain-brook. 
Flow-structure is most perfectly developed among the obsidians, rhyolites, 
tulsites and other acid rocks, of which it may be said to be a frequently 
"Conspicuous character (Fig. 9). In these rocks it is revealed by the parallel 
^irangement of the minute hair-like bodies and crystals, or by alternate 
^yers of glassy and lithoid material. The streaky lines thus developed are 
•^onietimes almost as thin and parallel as the leaves of a book. But they 
