OF THE HAWAIIAN ISLANDS. 437 



eastern Mauna Olokui has suffered a fracture similar to Konahuanui. The precipice left by 

 the rending apart of the mountain is steep and like that of Koolau, and at the bottom 

 is a plain of pahoehoe containing many cones and craters of comparatively recent origin. 

 Like Oahu, Molokai has been elevated above the sea considerably, probably from three to 

 five hundred feet. 



Lanai. — While Oahu, Molokai, and Hawaii present high precipices to the windward side, 

 Lanai slopes from a vertical wall on the lee side, that is to say, a break has occurred nearly 

 parallel with that on Oahu, but in this case the opposite side has fallen. No series of sound- 

 ings have been made off these islands, but the water is deep, and the break seems to extend 

 some distance below the surface ; but this is mere conjecture. 



Maid. — Much like Kaala at its western end, Maui in its great Haleakala closely resem- 

 bles Mauna Loa. Eeka is much broken through, and its ancient craters are now extensive 

 valleys. The whole mountain is so pierced that a tunnel a quarter of a mile in length 

 would connect the two sides. 



Hawaii. — The formation of this island has been traced in the successive outpouring of 

 its lava-streams, and we may turn at once to the consideration of the action of lava as 

 exhibited in the volcanoes of this group. 



Lava as a formative Agent. 



The lava rises to the surface and overflows, forming a bed of lava of a thickness propor- 

 tionate to the level of the ground ; the action is always intermittent, and as it diminishes 

 the lava cools ; another discharge increases the thickness of the layer, and where following 

 the first without the intervention of scoria?, ashes, or decomposition, may unite completely 

 with it, forming one bed. Where scoriae intervene, the two deposits are separated by a 

 distinct line. In this way we may have beds of eighty or a hundred feet in thickness, and 

 as the surface around the discharge is at first tolerably level, or has been made so by sub- 

 sequent eruptions, the lava will flow in all directions slowly, and form such uniform beds as 

 are seen in many places on the islands. Discharge succeeds discharge, and the mountain 

 is raised up around the orifice from which the lava flows. As the lavas are liquid, they 

 will flow away from the central opening, forming a gentle slope quite unlike the cinder 

 cones, where the material must remain nearly where ejected, until the angle of its sides 

 becomes too steep to retain additions. Consequently while cinder cones may have an angle 

 of 40°, lava cones seldom exceed 10°. The specific gravity of the lava exercises a consider- 

 able influence in determining the angle at which its streams will consolidate, and thus the 

 slope of the mountain. The Hawaiian lavas contain much iron and augite, and the dome 

 formed is quite flat. 1 



Each succeeding stream melts into the previous ones, unless a bed of cinders or soil inter- 

 poses their non-conducting properties, and in some cases I have seen recent flows so united 

 with the beds through which they pass, as to render it quite impossible to detect the point 

 where one ended and the other commenced. The more rapid the motion of a lava-stream, 



1 See Scrope on Volcanos, p. 131. 



MEMOIKS BOST. SOC. NAT. HIST. Vol. I. Pt. 3. Ill 



