3i8 PRINCIPLES OF STRATIGRAPHY 



occurs in injected masses such as dikes, sills, etc., where it is dften 

 very well developed. It is more marked in acid than in basic 

 rocks, especially in obsidians, rhyolites and felsites, etc. The flow 

 structure of basic rocks is more generally seen on a large scale 

 in the orientation of porphyritic crystals or the arrangement of 

 vesicles into line. 



Stratification of Floivs. A. succession of surface flows in a 

 given region will produce true stratification of the lavas in which 



Fig. 57. Columnar structure in basalt near Fingal's Cave, Island of Stafifa, 

 W. Scotland. 



each layer becomes in turn the top of the lithosphere at that point. 

 (See p. 697.) Such volcanic strata may be distinguished by differ- 

 ences in texture if not composition. 



Columnar Structure. This is a contraction phenomenon in 

 which prismatic columns commonly with six uniform faces form 

 on the cooling of the magma. In general the prisms form at right 

 angles to the enclosing walls. In dikes the prisms thus are hori- 

 zontal or nearly so, while in flows and intruded sheets the columns 

 stand upright. (Ex., Fingal's Cave [Fig. 57] ; Giants' Causeway, 

 etc.) Sometimes they are curved, as in Clamshell Cave, Staffa. 

 Often the columns or prisms are non-persistent through the bed, 

 but die out one into the other, with a wavy, irregular shape. (Geikie- 



