172 OUTLINES OF FIELD-GEOLOGY 



narrow, curved, irregular and starch-like as in the upper 

 basalt that overlies the columnar sheet in Staffa. 



Many ancient as well as modern lavas exhibit what is 

 called a "flow-structure," due to the irregular devitrifu a 

 tion and consolidation of the mass while still in motion. 

 (Fig. 67). This streaky structure sometimes shows re- 

 markable corrugation, indicating how the partially 



FIG. 68. Steam-holes in lava drawn out in the direction of the flow of the m* 

 (</) and becoming more and more flattened till on the line a b they are cum 

 pressed into mere streaks and give a fissile structure to the rock. 



solidified layers were driven onward by the motion of 

 the general body of the lava. By attending to the direc- 

 tion towards which these layers have been pushed, we 

 may now and then obtain satisfactory evidence as to the 

 quarter from which the streams of molten rock proceeded. 

 The upper part of a lava stream still in motion is often 

 a confused heap of rough blocks of slag, among which 

 rents appear opening down into the still red-hot mass 

 underneath. Many of these rents remain unclosed when 

 the lava comes to rest. If we imagine such a cracked 

 sheet of rock to have sand or mud laid over its surface, 

 the cracks would be filled up first, and if the sand were 

 brought in gradually, there might be time for it to 

 arrange itself in a stratified manner between the walls of 

 the fissures. No better evidence could be given that 

 the lava must have been poured out at the surface, and 



