LYONS ON LIFE HISTORY OF A VOLCANIC ISLAND. 97 



oomos from this ])oniliai'it.v. On tlic otlior hnnd such material is much 

 more likely (o have a rej;ularl.y jointed or columnar structure which may 

 render it liable to be quickly washed out by wave or stream erosion. 

 The Eainbow Fall near llilo owes its existence to the columnar structure 

 of the basalt, and it is only one instance of many. FingaTs cave is 

 a conspicuous illustration of the havoc the ocean makes with such 

 material. 



Again, volcanic cones are necessarily penetrated by vertical fissures 

 through which water readily penetrates to great depths. If the water 

 filters through such pores and crevices as it ordinarily finds in sedi- 

 mentary rocks, it loses its dissolved oxygen and carbonic acid before 

 sinking many inches into the rock. It is otherwise if it finds free 

 passages, and so its power to do chemical work at considerable depths 

 is greatly increased. The ducts which remain open in the center of a 

 great lava tiow^ — galleries, frequently six to ten feet in diameter and 

 extending continuously it may be several hundred yards, serve also 

 as subterranean conduits, and not unfrequently they determine the posi- 

 tion of streamlets destined to cut deep ravines. 



Volcanic rocks differ from most sedimentary and metamorphic rocks 

 (calcareous of course excepted) in the completeness of their destruction 

 by weathering. The sand used for building in the Hawaiian islands is 

 all brought from abroad. There is sand of course on the sea shore at 

 home, but it consists commonly of fragments of sea shells and corals. 

 In some places you will find black sand, but on examination it will 

 almost always prove to be simply fragments of the lava rock having no 

 enduring quality like the grains of an ordinary quartz sand. More 

 rarely you will meet with a sand composed of grains of chrysolite or 

 of magnetite, the most enduring minerals that occur in the lava, but 

 even these are chemically short-lived. The ultimate resultant, then, 

 from the disintegration of lava is not sand grains but impalpable dust, 

 so that our island cannot be expected to leave behind even a sand bank 

 to mark its site when the elements shall have done their work of 

 destruction upon it. 



As illustrating this destructibility of the volcanic material, I call to 

 mind an interesting formation I once found at tide level on Oahu. a 

 conglomerate consisting of beach w^orn lava pebbles cemented together 

 with calcium carbonate. The pebbles obviously represented the hardest 

 and most enduring portion of the country rock, yet I found them eaten 

 out from the cementing material which was one we customarily regard 

 as particularly destructible. What was most interesting in this case 

 was the fact that most of the pebbles had been eaten out in the interior 

 leaving a shell of more resistant material, this quality of resistance 

 apparently the result of the compacting of the shell by the pounding of 

 the pebbles on the beach. 



The lava when fresh is a sufticiently tough material. The geologist's 

 hammer often suffers in controversy with some of the compact varieties 

 of basalt, so long as these are sound — I was about to write new, but 

 I call to mind some very refractory boulders I have tried to sample that 

 came from eruptions that occurred certainly a thousand centuries ago. I 

 have seen rocks no older, and probably originally (]uite as hard, which 

 you might crumble to powder between your fingers. The first had never 

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