470 HARRY 0. WOOD 



practically straight line from a point a little west of this conspicuous 

 group to the summit plateau of Mauna Loa (see the photographs, 

 Plate VI, a and b). North of this lava ridge and east of the belt of 

 cinder cones (and to a less extent west of it also) is a comparatively 

 flat area. For a considerable distance to the northward toward 

 the summit, the surface of this stands at a lower altitude than the 

 summit of Puu o Keokeo, and its eastward extension from the 

 belt of cones, though variable, has a width of from two to three 

 miles. This makes a very conspicuous upland flat of very slight 

 grade on a broad dome surface which is itself of very gentle slope. 

 South of Puu o Keokeo the usual slope of the mountain is resumed. 

 At the southeast and east this broad, irregular flat passes imper- 

 ceptibly into the irregular, broken slope of the dome. Several 

 miles to the north of Puu o Keokeo the flat passes rather quickly, 

 though the region of transition is indefinite, into the somewhat 

 steeper slopes which rise to the summit plateau. Viewed from 

 the ridge south of this flat, Mauna Loa appears as a distinct 

 mountain until the genetic significance of the long belt of cones is 

 understood (see the photographs, Plate VI, a and b). 



This belt of cones marks an unmistakable major rift zone, 

 which joins at the summit with that leading down the northeast 

 flank of the mountain — a great crust fracture, as a whole slightly 

 curved and convex to the northwest, through the whole dome of 

 Loa. This stretches northward across the flat and bounds on the 

 west that part of it seen on this reconnaissance. 



SOURCES OF ERUPTION 



As was anticipated, the sources of the eruption of 19 16 were 

 found to lie in this major rift zone. Both the sources of earliest 

 outbreak, high on the dome, and the sources of flow lie in it. Both 

 were seen to constitute segments of the rift zone, and to be them- 

 selves rift traces in this zone. 



THE SOURCE OF EARLIEST OUTBREAK 



A long fissure marked by constant emanation of steam (or 

 fumes) was seen leading from near the summit plateau of Mauna 

 Loa down the south-southwest side of the upper slopes of the dome 



