96 MICHIGAN ACADEMY OF SCIENCE. 



Avheu voii see that this valley is parallel >vith a i»re<ipice marking a fault 

 by which the whole northern portion of the island has been cut off, to 

 disapi)ear in the deep sea except for a few points that remain as little 

 dots of islands just off the coast. The valley ai>peai-s to have been 

 formed by a si)littin<i of the mountain by a parallel fault. In this case 

 volcanic agency may have determined the catastrophe, since there has 

 been built at the foot of the precipice a little shelf of land just above 

 sea level cousistiug of obviously recent lava. But such an eru])tion 

 may as probably have been a consequence as a cause of the catastrophe. 



A still more striking illustration of the faulting I am speaking of is 

 found on the island of Maui, where the immense dome of Haleakala has 

 been split almost through its center, the windward half settling away 

 from the leeward portion so as to submerge the old coast line. ^?uch 

 a fault as this will determine without doubt the whole future history of 

 the mountain, erosive agencies henceforward centering about the two 

 great gorges thus produced on its o]>]»osite sides. One recalls imme- 

 diately the Ulowalu i)ass on \Vest Maui and the Xuuanu gap on Oahu, 

 recognizing at once the analogy in the conditions. It is noticeable in 

 each instance of faulting on this large scale that the catastrophe takes 

 }>lace in a region of abundant rainfall, and also on a coast exposed 

 to the trade wind swell of the Pacific ocean. 



Returning to our newly finished island — we might better speak of it 

 as an island on which work has just been begun — possibly it has 

 already suffered from faulting, certainly erosive agencies have begun 

 their work. The task these have undertaken will not be finished as 

 long as a vestige of the mass remains visible. I need saj' nothing of 

 the general principles which govern the action of these. There are, 

 however, peculiarities in the nature of the mass that is attacked which 

 may materially modify the results of their operation. The first point 

 to note is the heterogeneous character of volcanic accumulations, even 

 in the case of the basalt volcanoes of which I am especially treating. 

 The lava of which such a volcano is built up often appears in section 

 almost like a stratified rock. There is not greater difference in the 

 thickness of the successive beds than in the case of stratified rock of 

 sedimentary origin. The difference, however, in hardness and in re- 

 sisting power toward disintegrating agencies is often very great. As the 

 fresh lava varies extieuudy in vesicularity, so. after it has been com- 

 pacted by pressure au<l the action of heat, there is no uniformity in its 

 physical character or in its chemical reactions. 



Much fragmental material is ejected even from basalt volcanoes, piled 

 u]) into cinder cones some of which will become buried in the bedded lava 

 to leave material that will be (piickly washed out wheu erosion begins 

 its work. Near ITilo there is a pretty illustration of uiy jtoint in the 

 natural arch left by the washing out at the base of a pronumtory of 

 the material of an old buried tufa cone. Observe that fragmental 

 nuiterial will be especially abundant in the vicinity of the principal 

 vent, i. e., in the very core of the mountain, so that we must not be 

 surprised to find this jiart gouged out as it Avere while the peripheral 

 ])ortions of the cone i-euiain coui])aratively intact. 



The uiaterial of laccolites and of dykes may be expected to have much 

 higher resisting powers than tlie ordinary lava. The name dyke, indeed, 



