xii i;i 



they may frequently be obtenred to be pulled out int.. 

 oval or almond (amygJaM*/) shapes in the direction of 

 the motion (Fig. 68). Indications of this kind mark 

 the flow of lavas in all geological periods, and should 

 always be noted when they occur, for they may help to 

 show from what quarter the lava came, and where there- 

 fore the vent of emission should be looked for. 



During their cooling and contraction, sheets of lava 

 acquire a jointed v. uch as has already been re- 



ferred to. The chief set of joints usually starts from the 



: . 



cooling surfaces, that is from the top and bottom of the 

 sheets ; but other systems of " fissures of retreat " may be 

 developed more or less at right angles to these, so that the 

 lava is divided into vertical masses, which may be further 

 cut into segments. The most perfect joint-structure is 

 that known as the columnar or basaltic, where a rock is 

 built up of slender pillars, having five, six, or an irregular 

 number of sides. This structure is conspicuous in many 

 basalt-sills among the Western Isles of Scotland. Some- 

 times the pillars are long and parallel as in the columnar 

 part of Stafla (Fig. 65) ; in other cases they are short, 



