'^hap. in 
STRUCTURES OF LA VAS 
2 
Some lavas, more especially among the basic series, assume in cooling a 
^olurnnar structure, of which two types may be noticed. Tn one of these the 
'Columns pass with regularity and parallelism from the top to the bottom of 
^ bed (Figs. l7l, 225). The basalt in which Fingal’s Cave, in the isle of 
^taffa, has been hollowed out may be taken as a characteristic example 
266a). Not infrecpiently the columns are curved, as at the well-known 
^'lam-shell Cave of Statfa. In the other type, the columns or prisms are 
not persistent, but die out into each other and have a wavy, irregular shape, 
somewhat like prisms of starch. These twm types may occur in successive 
slieets of basalt, or may even pass into each other. At Staffa the regularly 
FiCi. ll._View at the entrance of the Sviiiofjord, Faroe Islande, illustrating the terraced forms 
a.s.sunied hy basic lavas. 
The islanil on the left is Borii, that in the centre Viderii, and that on the right Svino. 
^joliuinyjj^j. lied is immediately overlain with one of the starch-like character. 
he columnar structure in either case is a contraction phenomenon, produced 
' niiiig the cooling and shrinking of the lava. But it is difficult to say what 
special conditions in the lava were required for its production, or why it 
sometimes have assumed the regular, at othei’s the irregular form. 
^ 'nay be found not only in superficial lavas but in equal pei’fection in 
®°nre dykes and intrusive sills or injections, as among the Tertiary volcanic 
’■’neks of the island of Canna (Figs. llOV and 308). 
^he precipitation of a lava-stream into a lake or the sea may cause the 
nuter crust of the rock to break up with violence, so that the still molten 
naterial inside may rush into the water. Some basic lavas on flowing into 
"'ater or into a watery silt have assumed a remarkable spheroidal sack -like 
