102 J. D. Dana — Eruptions of Kilauea and Mt. Loa. 



that the region flowed over and making aa was one having 

 more or less of subterranean moisture, since only moisture 

 could produce the partial cooling required ; not a superfi- 

 cial stream of water that the lava could evaporate and so put 

 out of its way, but deeper and more widely spread moisture ; 

 and not too much for the quiet work of molecular imbibition 

 and thereby of cooling and fracturing, with sometimes a " tre- 

 mendous roaring like ten thousand blast furnaces." The aa 

 near Hilo, of which I have spoken, was over a valley depres- 

 sion beneath which such an amount of moisture may well have 

 existed. Another was along the foot of the meeting slopes of 

 Mt. Loa and Kilauea, west-southwest of Kilauea, But my own 

 observations were too brief to authorize a positive opinion as 

 to the influence of the form of the surface in these cases ; and 

 in others, according to the descriptions, the surface covered by 

 the aa is not always depressed. 



There must be more or less moisture in the dark recesses of 

 Mt. Loa. The cold summit will find enough in the air to con- 

 dense at most seasons. And the percolating rains must keep the 

 recesses damp and even make standing water wherever the 

 rocky layers favor it. With subterranean moisture a hundred 

 yards more or less beneath the broad lava-bed, the generated 

 vapors would ascend into and through the liquid mass, cooling 

 it thus from below, yet not so much the hotter bottom which 

 receives new supplies of lava, as the portion above. The part 

 solidified would become shattered or broken up by the tearing 

 steam and by contraction from cooling ; and, at the same time, 

 the flow at bottom would displace and tumble together the 

 great and small masses, giving the pile height because of the 

 jagged forms of the blocks and the cavernous recesses left 

 among them. This view appears to meet the demands of the 

 facts I have observed, and all others so far as they have been 

 published. But I present it only as a suggestion. 



On this view an aa stream is literally an arate or ploughed 

 up lava-stream ; a stream ploughed up from near its bottom, so 

 that, although vesiculated, the surface vesiculation fails, as was 

 well shown in the stream of 1880-81 near Hilo, and in all the 

 other cases I examined. 



Dome-shaped bulges in a cooled lava stream would naturally 

 be common over the pahoehoe part of it where the stream 

 begins to pass to the aa condition. 



The bomb-like masses, concentric in structure, observed over 

 aa streams (xxxiv, 364, and in the figure on page 100), varying 

 from an inch to a mean diameter of 10 feet, appear to be pro- 

 duced through the rolling movement in the forward portion of 

 the advancing aa stream, clue to friction at bottom (p. 101). 

 They are often a heap of fragments of scoria inside with a 



