chap. Ill 
STRUCTURES OF LA VAS 
23 
tile constitution of the magma from which the la^'as issued — variations 
"diich already existed before the discharge of these lavas, and which showed 
themselves in the successi\'e outflow of basaltic and doleritic material during 
the eruption of what was really, as regards its appearance at the surface, one 
Continuous stream of molten rock. It is impossible to account ior such 
'’ariations in the same sheet of lava by any process of differentiation in 
the melted material during its outflow and cooling. Analogous variations 
Occur among the basic sills and bosses of the lertiary volcanic series of 
Ih'itain. These, as will be more fully discussed in later chapters, indicate 
^ Considerable amount of heterogeneity in the deep-seated magma from which 
the intrusive sheets and bosses were supplied (see vol. ii. pp. 329, .t42). 
Fig. 9. — Flow-struftiirt in Rhyolite, Aiitriiii, .slightly reduced. 
It is a common error to assume that flow-structure is a distinctive 
^^araeter of lavas that have flowed out at the surflice. In reality some 
^ the most perfect examples of the structure occur in dykes and sills, both 
acid and basic rocks. Innumerable instances might be quoted from 
Ih'itish Isles in support of this statement. 
Altliough, in the vast majority of cases, tbe presence of flow -structure 
^^ay be confidently assumed to indicate a former molten condition of the 
in which it occurs, it is not an absolutely reliable test for an igneous 
K.xperiment has shown that under enormous pressure even solid 
’Petals niay be made to flow into cavities iirepared for their reception. 
