22 
VOLCANIC PRODUCTS 
BOOK I 
generally show interruptions and curvatures, and may be seen to bend round 
larger enclosed crystals, or to gather into eddy-like curves, in such a manner 
as Auvidly to portray the flow of a viscous substance. These lines repi'eseut 
on a minute scale the same flow-structure which may he traced in large 
sheets among the lavas. The porphyritic crystals and the spherulites are 
also drawn out in rows in the same general direction. Sometimes, indeed, 
the spherulites have been so symmetrically grouped in parallel rows that 
they appear as rod-like aggregates which extend along the margin of a dyke. 
Among lavas of more basic composition flow-sti-uctuie is not so often 
well displayed. It most frequently shows itself by the orientation of 
porphyritic felspars or of lines of steam -vesicles. Occasionalh", however, 
sheets of basalt may Ije found in which a distinct streakiness has been 
developed owing to variations in the differentiation of the original molten 
Fig. 8. — Variolitk' or Orbicular structm-c, Napoleoiiite, Corsica (iiat. size). 
magma. A remarkable and wide-spread occurrence of such a structure 
is met with among the Tertiary basalt-plateaux of the Inner Hebrides and 
the Faroe Islands. In the lower parts of these thick accumulations of suc- 
cessive lava-sheets, a banded character is so marked as to give the rocks the 
aspect of truly stratified deposits. The observer, indeed, can hardly undeceive 
himself as to their real nature until he examines them closely. As a full 
description of this structure will be given in a later chapter, it may suffice 
to state here that the banding arises from two causes. In some cellular 
lavas, the vesicles are arranged in layers which lie parallel with the upper 
and under surfaces of the sheets. These layers either project as ribs or 
recede into depressions along the outcrop, and thus impart a distinctly 
stratified aspect to the rock. More frequently, however, the banded 
structure is produced by the alternation of different varieties of texture, 
and even of composition, in the same sheet of basalt. Lenticular seams of 
olivine -basalt may be found intercalated in a more largely crystalline 
dolerite. These differences appear to point to considerable variations in 
