478 HARRY 0. WOOD 



direction almost exactly magnetic north 1 (see Plate VI, a). This 

 fissure, or system of fissures, is practically uninterrupted and, 

 except for a short curving segment at the north end (see Plate VI, c), 

 it follows a straight line. It lies wholly within the limits of the 

 great rift zone, but its course is slightly oblique to the general 

 trend of that, which is about N.N.E. (see Plate VI, b), and this 

 suggests a major shear through the mountain. It ends on the 

 north at an old red cinder cone at an elevation of about 7,480 feet 

 above sea-level, while at the south its point of interception with 

 Puu o Keokeo is at an elevation of about 6,600 feet (see maps, 

 Plate I and Fig. 1). 



Besides this chief rectilinear fissure, or primary group of fissures, 

 there are a great many secondary cracks, running roughly parallel 

 with the system, especially on the west; on the east there are few. 

 Many of these opened after the chief outpouring of lava was over, 

 for they traverse the fresh flows. (It is notable, moreover, that 

 earthquakes continued to increase in number and energy until after 

 the eruption began to decline definitely.) Many, however, traverse 

 the older surface neighboring the source; and here they appear as 

 consistent extended fissures in the more or less solid basalt of the 

 mountain, but also there is noted a tendency for the fissuring to be 

 continued from one crack to another through offsets en echelon (see 

 Fig. 2). 



Though miniature dislocations have resulted necessarily, there 

 is no observable tendency to any general vertical dislocation; but 

 the repeated evidences of offsets en echelon strongly* suggest a general 

 horizontal shear. There are, however, no sufficiently well-indicated 

 and extended landmarks, surface features, or structure lines to 

 afford a real test or proof of this. Where these cracks traverse the 

 old surface and where they cut the new flow, there generally is no 

 evidence of gas outrush, bulging, or fumarolic action, or any evi- 

 dence of heat emission. 



Altogether, as study progressed the conviction gained force 

 that the amount of rending of the mountain dome here seems out 



1 The general magnetic declination in Hawaii is about N. io° E., but large local 

 variations make the application of corrections so uncertain that the direct magnetic 

 reading is given here by preference. 



