jaggar: aphrolith and dermolith 279 



pear to be blown by expanding gas, for the axis is almost tubular 

 in its coarser vesicularity. The spasmodic renewed advance of 

 a pahoehoe flow is frequently characterized by pushing out 

 from under a skirt of crust, in contrast to the aa flow which 

 appears to solidify from below upward and to advance by over- 

 riding its already solidified lobes. Pahoehoe lava is variously 

 mentioned in the text books as fluent, smooth, ropy, or corded 

 lava. 



Visually the two types of lava, in mobility and temperature, 

 appear much alike while incandescent and flowing rapidly as 

 golden streams. With the beginnings of solidification, however, 

 the difference is pronounced, pahoehoe forming skins and aa 

 forming knobs and cinders like a bed of hot coals. The cool- 

 ing front of an aa flow is full of flames through the apertures of 

 the blocks. The front of a pahoehoe flow shows no flames de- 

 tectable by eye. The aa flow at its front is in a brittle semi-solid 

 condition, slabs rifting away and falling forward from a mass 

 beneath that resembles hot iron. Pahoehoe, on the other hand, 

 bellies its crust forward, escapes as a viscous pudding and this 

 in turn skins over, the whole movement resembling the ad- 

 vance of candy or tar. 



The explanation of the physico-chemical difference between 

 the two types remains to be discovered, as chemical analysis 

 in the same region has as yet revealed no distinction. It seems 

 probable that the quantity of confined gas, in solution or in 

 bubble form, for each unit of volume of melt, controls the mode 

 of freezing. Possibly the gases are nearer equilibrium in pahoe- 

 hoe than in aa. The heat equation plays an important role, 

 and this involves reaction between the gases as well as their 

 oxidation in air. Gas expansion may be more rapid in aa and 

 so induce internal solidification. Furthermore, there are enor- 

 mous differences in the state of oxidation of the iron at the 

 moment of cooling, and as j^et we know nothing of the progress 

 of crystallization in the field. With so many variables there is 

 no cause for wonder that the distinction is as yet unexplained. 



Between pahoehoe and aa flows in Hawaii there are unquestion- 

 ably gradations of many appearances and perhaps of many 



