VI 



WAINUNU TABLE-LAND 85 



inclined at a large angle from the perpendicular. 1 He also refers 

 to some prisms of a grey basalt exposed just below the Wailuku 

 Falls near Hilo in the large island of Hawaii which were 8 feet 

 in diameter and were surmounted by others only I to 4 feet across. 



The basalts of the Wainunu table-land are blackish and non- 

 vesicular, with a density of 2^87 to 2'go. They all carry olivine 

 and microporphyritic plagioclase, and display a little interstitial 

 glass, and the felspar-lathes are usually in plexus-arrangement, 

 being stout and often showing twin lamellae. But the rocks 

 exhibit important variations in different localities as regards the 

 amount of olivine, the length of the felspar-lathes, the presence or 

 absence of the ophitic character, &c., and they are grouped in 

 different genera of the olivine class (i, 13, 25, 33). Probably the 

 type of genus 25, with scanty olivine and granular augite, would 

 prevail. 



From the varying size of the felspars of the groundmass it is 

 apparent that the flows are not all of the same character. At 

 Masusu, where the rock is doleritic in texture, they average from 

 25 to '3 mm. in length. A mile further north, they are about 

 -17 mm. long, and two miles more to the north they average only 

 *i mm. in length. It is probable that a semi-vitreous basaltic 

 andesite (spec. grav. 273), that shows no olivine and is referred to 

 the porphyritic sub-genus of genus 9 of the augite-andesites, which 

 is exposed in the stream-courses near the base of the dacitic 

 mountains of the interior, is the product of a later eruption. 

 Occasionally one finds, as at Thongea in the Wainunu valley, a 

 basalt rich in olivine (spec. grav. 2*95), the felspars of the base 

 averaging 'i mm. in length. It may be remarked here that one 

 cannot draw a sharp distinction between the basalts of this region 

 and those of the adjacent eastern slope of Seatura. Their specific 

 gravity is about the same (2-87 to 2*90) ; but the coarse texture of 

 the Masusu basalts did not come under my notice in the last 

 locality, where the felspars of the groundmass average *i8 mm. in 

 length or about two- thirds the length of those of the Masusu 

 rocks. 



By referring to the section across this part of the island, it will 1 

 be observed that the basaltic lavas of this table-land must have 

 issued from some fissure near the south side of the base of the 

 Ndrandramea mountains. In crossing the head of this plateau 



1 A similar arrangement was observed in the columnar basalt of Kauai 

 in the Hawaiian Islands. It is presumed that these Hawaiian flows are sub- 

 aerial. 



