26 
VOLCAN/C PRODUCTS 
BOOK I 
or pillow-like structure, the spheroids being sometimes pressed into shapes 
like piles of sacks. A good instance of this structure occurs in a liasalt at 
Acicastello in Sicily.^ A similar appearance will be described in a later 
chapter as peculiarly characteristic of certain Lower Silurian lavas associated 
with radiolarian cherts in Britain and in other countries (Fig. 12). 
It piobably seldom happens that a solitary sheet of lava occurs among 
non-volcanic sedimentary strata, with no otlier indication around it of former 
volcanic activity. Such an isolated record does not seem to have been met 
with in the remarkably ample volcanic register of the British Isles. The 
outpouring of molten rock lias generally been accompanied with the ejection 
Fig. 12.— Sack-like or pillow-form structure of basic lavas (Lower Silurian), Beunaii Head, 
Ballantrae, Ayrshire. 
of fragmentary materials. Hence among the memorials of volcanic erup- 
tions, while intercalated lavas are generally associated with sheets of tuff, 
bands of tuft may not infreipiently be encountered in a sedimentary series 
without any lava.^ Instances in illustration of these statements may be 
culled from the British I’aheozoic formations back even into the Cambrian 
system. 
A cbaracteristic feature of some interest in connection with the flow of 
lava is the efiect produced by it on -the underlying rocks. If these are not 
firmly compacted they may be ploughed up or even dislocated. Thus the 
tuffs of the Velay have sometimes been plicated, inverted, and fractured by 
1 See Prof. G. Platania in Dr. Johnston-Lavis’ South ltalia7i Volcanoes, Naples (1891) ii 41 
and plate xii. ' ^ 
