LYONS ON LIFE HISTORY OF A VOLCANIC ISLAND. 99 



whicli foi'iii on tlio Avindwnrd si<le will deposit rnin only on tliat side, the 

 leeward side noi only leeeivin^' no rain but leniainin};- generally free 

 from clond. T 'nder these conditions the valleys on the leeward flank of 

 the mountain Avill retain their canyon form even in their upjjer portions. 

 If, as in the Hawaiian islands, occasional heavy rains occur near sea 

 level on the leeward as well as the windward side of an island, the 

 valleys may be broadened somewhat in the lower part, but these 

 copious occasional rains have little effect after all in disintegrating 

 the lava, which in the intervals lies exposed to a cloudless sun in a 

 region bare of shrubbery. 



If the mountain is less than 5,000 feet high, clouds will lie much of 

 the time over the whole summit and a portion of the rain will fall 

 on the leeward side of the mountain. If the elevation is barely :5,000 

 feet much the greater pari of the rain will be discharged on the 

 leeward side. There will be in each case a point of maximum rainfall, 

 and here we shall find evidence of greatest activity in the work of 

 erosion. 



The island of Hawaii illustrates how even in the case of very lofty 

 mountains, there may be an area of excessive precipitation on what 

 would seem to be the sheltered portion of the island. There is here 

 possibly a key to the a]»parent anomaly of the extraordinary weather- 

 ing of the leeward side of the Kaala mountain mass on Oahu, but of 

 this more hereafter. 



The last stage in the life history of a volcanic island will be naturally 

 a prolonged and uneventful one. Denudation has proceeded so far that 

 little or no condensation of moisture is occasioned by the insignificant 

 remaining ridges, which therefore weather very slowly, crumbling, 

 however, little by little to dust, and carried finally by wind and rain back 

 to the ocean from which it emerged. 



I have recited with some fullness of detail this story of the normal 

 life history of a volcano because it is only by a familiar acquaintance 

 therewith that it is possible to read backward the same history from 

 any stage except the very latest in a study of the ruins that remain. 

 The data for such a study will not, however, be complete if we ignore 

 the possibility that there may have been important changes in sea 

 level, and often coincidently in climatic conditions, due to geological 

 events in other parts of the world. The evidences of such changes will 

 be easily enough found if we are alert to look for them. 



Again there is the possibility — almost the certainty — that there will 

 be repeated recurrence of volcanic action, which wilf build anew amid 

 the ruins of the old structure but with no reference to its original plan. 



The problems presented by the island of Oahu are peculiarly com- 

 plicated in that the present island has been formed by the fusion of two 

 volcanoes, one notably older than the other, and further because of the 

 repeated and numerous volcanic outbreaks, not confined, as usual, 

 to the peripheral portion of the island. 



The paper of Prof. Hitchcock seeks to arrange in their chronological 

 sequence the principal events in this very interesting history. Con- 

 fessedly it leaves much for future students to verify and fill out. It 

 presents a skeleton outline of what science has definitely established, 

 giving in most cases convincing reasons for the conclusions drawn. 



