1 6 Plant !>ivasio>i on Lava Flows. 



whole mountain is a gigantic mass of these lava streams which 

 radiate on all sides from the summit, 13,675 feet, to sea level. 

 The later flows have generally arisen from sources below the 

 summit. The different flows have had irregular courses. Many 

 of them in flowing over older streams have left areas of various 

 sizes of the older flow surrounded on all sides b}' the newer flow, 

 without apparentl}' harming the vegetation of the resulting island 

 to any appreciable extent. The surrounded areas, known to the 

 natives as "kipuka," may be above or even below the surface of 

 the surrounding flow. Fortunately for a study of this sort the 

 age of man}- of these flows is known. 



The lava flows are of two kinds, generally simply described 

 as the smooth or slaggy, and the rough or scoriaceous; but as 

 these regions differ so much from the country generally traversed 

 by botanists, I quote the fuller word picture of Dana.- "There is 

 the ordinary smooth-surfaced lava called pahoehoe, the term sig- 

 nifying having a satin-like aspect. The surface of the lava cooled 

 as it flowed. Through one means and another the surface is 

 u.sually uneven, being often wrinkled, twisted, ropy, billowy, 



hummocky, knobbed, and often fractured The other most 



prominent kind of lava stream is the aa. The aa streams have no 

 upper flow-like surface; they are beds of broken up lava, the break- 

 ing of which occurred during the flow. They consist of detached 

 masses of irregular shapes, confusedly piled together to a height 

 sometimes of twenty-five to forty feet above the general surface. 

 The size of the ma.sses is from an inch in diameter to ten feet and 

 more. The lava is compact, usually less vesiculated than the 

 pahoehoe, not scoriaceous; but externall}' it is roughh- cavernous, 

 horribly jagged, with projections often a foot or more long that 

 are bristled all over with points and angles. In some cases ragged 

 spaces extend along planes through the large masses, like those 

 of the exterior." Both kinds of lava may be represented in the 

 same flow, either in different parts or close!}- associated. Manj^ 

 hundreds of these two classes of flows were passed over during the 

 excursion. 



The first flow of known date visited was that of 1859. This 

 flow was followed from a point west of Puuwaawaa down to where 

 it crosses the Government road. It is composed of both aa and 



^Dana, J. L).: The Characteristics of Volcanoes, 1891, p. 9. 



