442 W. T. BRIGHAM ON THE VOLCANIC PHENOMENA 



collapsed bubbles, as Mr. Scrope and others seem to think. They have not been formed by a 

 Ming in of the top of the mountain, but the process is this: When the lava subsides in the 

 tube after a surface overflow, it leaves a well or depression, which will have perpendicular 

 walls. A violent commotion in the lava at the bottom of this pit at a recurrence of volcanic 

 activity will crack and dislodge portions of the wall, which will be swallowed up in the 

 melted mass. The lava may again rise and overflow the rim, and its heat will both melt 

 and crack the walls, and thus the crater of an active volcano of the Hawaiian type con- 

 stantly increases in diameter. "When the lavas no longer overflow, they may yet reach the 

 base of the encircling wall and remove the debris, which falls, maintaining the perpendic- 

 ularity of the cliffs. This has been the case at Kilauea, and the fragments broken from the 

 upper walls now form a low ridge some distance from where they fell, having been trans- 

 ported by the fiery currents. The natives declared to Mr. Ellis, on his first visit to the crater, 

 that it had grown much since ancient times. 



Neither in these craters nor anywhere on the mountains can I see any signs of '•' a single, 

 sudden swelling up of previously formed horizontal beds of lava and scoria3 into a hollow 

 bladder." 



Influence of constant Winds. 



The north-east trades blow during nine months of the year with considerable force, and 

 their influence is seen in the tufa cones which always are higher and broader on the south- 

 west, as the comparatively light tufa is easily blown by the winds. But in the ridges them- 

 selves there is a curious feature which probably ow T es its origin to the cool- 

 ing influence of these winds. The general direction of all the spurs is 

 from the north-east. The windward sides of the islands usually present 

 perpendicular precipices, or short steep ridges where the islands are 

 broken, as in Oahu and Molokai, while Hawaii presents to the force of 

 the waves, unbroken bv anv protecting coral reef, a precipice several 



Makapuu Ridges. •> •> L a 1 l 



hundred feet above, and at least as many below the sea level. 

 A direct result of a constant direction of the wind is the more rapid decomposition of 

 the walls of the smoking crater by the action of the sulphurous vapors, causing a crumbling 

 of the walls when the action is long continued. 



Earthquakes not a necessary concomitant of Eruptions. 



It has been remarked that the eruptions of Mauna Loa are wholly unaccompanied by any 

 great commotion of the earth. While earthquakes do occur on the Hawaiian Islands, they 

 are never severe, and seldom are noticed during an eruption. They seem to confirm Mallet's 

 theory of vibrations, as they almost always radiate from the solid axis of the centre of an 

 island. A slight shock was felt on Oahu in the winter of 18G5, when the vibrations were 

 distinctly felt from the mountain to the sea on all sides of the island. 



[The remarkable difference in level between lavas in the same mountain will be con- 

 sidered in the Topography of Kilauea.] 



Sea Waves. 



It is not unusual, during littoral or submarine eruptions, for the sea to be agitated by 

 great and unusual tides. Such sea waves have not attended the recorded eruptions of the 



