278 jaggar: aphrolith and dermolith 



in plan. The distinctive feature of typical aa, however, is the 

 shrinkage of the surface into units from a few centimetres to 

 several metres in diameter, each unit being complete in itself 

 and commonly free from broken surfaces. Such lava is spoken 

 of in text books which have not adopted the Hawaiian word, 

 as fragmentary lava, block lava, or scoriaceous lava, and in 

 the southwestern states the Mexican word "malapai" or "mal- 

 apis" is used for the vast fields of aa which there occur. In 

 the Cordilleran belt and elsewhere great bodies of ancient aa 

 are called pyroclastics, breccias, and agglomerates indis- 

 criminately, along with explosion products that are of totally 

 different origin. Aa makes a pyroclastic where igneous ma- 

 trix is apt to be identical with the contained fragments, and the 

 fragments are of irregular lumpy outline. 



The pahoehoe lava of the Hawaiians, on the other hand, is 

 distinguished by smooth surfaces which result from the for- 

 mation of a glassy membrane or skin. The mode of congelation 

 of olivine basalt magma advancing as a pahoehoe flow is very 

 strikingly different from that of an aa flow. The surface of the 

 lava lake of Kilauea in all stages of its crusting is typically 

 pahoehoe. As the heat is radiated from the surface, solidifica- 

 tion forms a dermal layer made of filaments of glass remnant 

 from expanding bubbles filled with gas, and these show no 

 tendency to burst explosively, but incessantly crowd into the 

 interstices of the stretching and rending layer of filaments of 

 previous formation. The result is a skin not usually gas tight 

 and exhibiting extraordinary diversity of texture. Solidifica- 

 tion proceeds from without inwards, the skin reaches various 

 thicknesses and then folds, and the folding distributes itself 

 somewhat like the moraines of a glacier with festoons drawn 

 downstream farthest along the middle line and retarded at the 

 two sides. At the sides there are lengthwise curtains produced 

 by the longitudinal drag between resistant cooling margins and 

 the festooned stream in the middle. At the front, toes of the 

 molten fluid push out from under the skirts of the flow in 

 I peculiar tuberous bodies, which tend to congeal on all sides 

 J and may even lift their tips vertically and solidify. They ap- 



