334 HARRY 0. WOOD 



bulge out a little without spurting or jetting. No cracks could be 

 made out on these freshly bared, red-hot surfaces. They exhibited 

 every appearance of a viscous continuum. Once they had crusted 

 over, however, the tendency to crack and bristle again became 

 noticeable. At this place the fragmentation and texture-forming 

 of these rough-surfaced blocks suggested very strongly the breaking 

 and cracking of candy pulled too long. There was no observable 

 gas action. 



The whole effect here was one of creep, or overrunning, with 

 the plane of maximum rate of flow intermediate between the top 

 and the bottom. The action was that of a very viscous, fluid or 

 plastic, substance, flowing very slowly and exerting subsurface 

 traction upon a surface crust too stiff to draw or pull much. 



It is thought that the matter was here cooled to a point where 

 it could still flow, or creep along, under blanketing, but once in 

 touch with the air it set so stiffly that any further strain frag- 

 mented it. Pieces artificially broken away from the bulging 

 surfaces cooled without further fragmentation, and without the 

 development of a-a-texture 1 on those surfaces of the fragments 

 which were glowing when broken away. These exhibited the 

 rough fracture of cold basalt. The rough, pointed, and edged 

 texture of the natural a-a surface was seen in some cases to be due 

 to the drawing out of points and the shaping of rough edges as the 

 blocks were tilted and rotated away from each other, and from the 

 plastic matrix within, while the forward bulging movement was 

 taking place. 



The mechanism of the formation of a-a has been considered a 

 very involved and complicated process, and the problems suggested 

 by it difficult of solution. The writer does not consider that the 

 observed action described above will serve to explain all cases and 

 details. But it does, he considers, indicate strongly that a-a 



1 A word concerning a-a-texture may not be out of place. As seen in Hawaii a-a 

 is not only block-lava, heaped flows of piled blocks and fragments of various shapes 

 and sizes, but each of these fragments is a separate unit, and its whole exterior surface, 

 in most instances, is characterized by an exceedingly rough aggregation of points, 

 edges, blades, spikes, knobs, etc., produced there in the process of the spalling and 

 transport of the blocks by the drawing, and possibly sometimes by spurting, of the 

 red-hot viscous basalt. 



