COMPARISON OF MOHOKEA WITH HALEAKALA 489 



ual descent from the axis connecting the outer to the inner angles of the 

 elbow in both directions. Where the outermost edges of the two arms are 

 reached there is a more rapid descent, commencing with 6,500 feet at 

 Koolau and 7,600 feet at the Kaupo gap. 



The similarity between the Mohokea and Haleakala calderas consists 

 in the presence of steep escarpments at the lower edges of the floor, and 

 both are unlike the typical examples (Kilauea), in that they are open on 

 one side, not encircled by a cliff. Haleakala could be conceived of as 

 consisting of two smaller calderas united along the axis of the elbow; 

 or it might be imagined as formed by the splitting of the mountain and a 

 separation of the two parts, the space between being filled by later dis- 

 charges. 



The gaps are each continued in broad valleys to the sea. Koolau 

 merges into the Keanae valley, reaching the sea at the village of that 

 name, 9 or 10 miles long. This valley is now crossed transversely by an 

 aqueduct fully 1,200 feet above the sea, carrying water for irrigation pur- 

 poses to the sugar plantations of central Maui. The Kaupo gap extends 

 to the sea in a similar manner, taking its name from the locality. These 

 two streams of lava are larger than any now known elsewhere in the 

 archipelago. If the lava should accumulate enormously in Kilauea, and 

 one stream flow south to Punaluu and the other break through the bar- 

 rier to the edge of Puna and thence to the sea, the topography of the 

 caldera and its outflows would be very suggestive of Haleakala. 



Two views of Haleakala are presented. Plate 65 is a restoration — an 

 attempt to show the appearance of the caldera as if one were situated in 

 a balloon a thousand feet above the highest point. It is reduced from a 

 painting by E. Bailey, based upon W. D. Alexander's early map. Plate 

 66 is a photograph of the south wall of Kaupo, with views of some of the 

 smaller craters inside the pit. 



Phases in the Development of Hawaiian Calderas 



It is easy to speculate on the relations of the several Hawaiian 

 calderas. 



At first there is a simple crater discharging lava from the summit of a 

 dome. 



Secondly, the lava is not produced in sufficient quantity to flow over 

 the margin ; the opening is sealed, and then the outermost crust breaks up. 

 The crust is too vast to be absorbed; blocks of it will be elevated; other 

 sections will be absorbed, and the outer wall on the makai side may give 

 way. There will be discharges on the lower side. This may be the 

 Mohokea stage. 



