37G w - T. BRIGHAM ON THE VOLCANIC PHENOMENA 



The Wailuku, the largest river of Hawaii, empties into the sea at Hilo. Its sources are on 

 the south-eastern slopes of Mauna Kea, and it forms the boundary between Kea and L6a. 

 Occupying such a position, it has received the lavas of both mountains, and exhibits in its 

 rocky bed many a record of conflicts between fire and water. One of these " writings on 

 the wall" discloses the interesting fact that Kea poured forth lava after the surface was 

 sufficiently decomposed to retain the streams which supply the Wailuku. This is shown in 

 several places, although it may still be one and the same stream, and it must not be inferred 

 that the stream of water which produced the effect on the melted lava was a temporary 

 flood from the snows melted by the eruption. When the snows melt on Mauna Loa no 

 stream from them reaches the base of the mountain, but the water sinks at once into the 

 porous rock where the snows lie, and to supply the bed of the Wailuku even temporarily, 

 Mauna Kea must have been subject to the decomposing forces of many ages, unless it be 

 supposed that it was originally built of a more compact material, which certainly is not 

 shown by any examination that has yet been made. 1 



The water has often flowed over heated beds of basalt, and the consequence has been the 

 formation of columns radiating from the bottom of the stream. Often where falls occur, 

 this columnar structure is beautifully exhibited as Gothic archways, from whose apex the 

 torrent pours into a basin surrounded by these curved and broken, half-sunken prisms, 

 black and prominent amid the white foam of the falls. Where the lava has poured into 

 the water, deposits of black volcanic gravel are usually formed from the fracture of the 

 lava, but such deposits are from their nature peculiarly liable to be removed by the action 

 of the water, and it is only where the course of the stream has been changed that they are 

 found in situ. 



From the much worn condition of the lava over which the river flows it might be sup- 

 posed that the rock was quite soft, but the great attrition is partly to be attributed to the 

 harder fragments of dolerite brought down from the heights above by the freshets, which 

 scrape and grind the bed of the stream to a great extent, and are themselves rounded when 

 they reach the sea. Under the falls, which are sometimes a hundred feet or more in 

 height, deep pools are worn, into which it is the delight of the Hawaiian youth to jump 

 from the dizzy verge of the fall above. 



The Anuenue, or Rainbow Falls, are about a mile from the sea, and beneath the sheet of 

 water which falls more than a hundred feet is seen another result of the sudden chilling of 

 heated — not melted — basalt. The columns at the point of contact with the water are at 

 right angles to the surface, but curve regularly below, and the surface is much harder than 

 the lower portions, so that while quite perfect near the top where the wearing action of the 

 water is considerable, they are completely washed away below, forming a cave of some depth 

 beneath. It has been suggested that this and similar caves beneath water-falls were simply 

 bubbles in the lava-stream, and that the curved prisms owe their origin to the more rapid 

 cooling of such a cavity, even without the agency of water. But wherever columns of 

 basalt occur around such bubbles, they are always at right angles to the surface of the 

 bubble, that is, in a section, they form radii and not arcs, while in the three or four water- 

 falls examined they exhibited the structure represented in the diagram. 



1 No foreigner seems to have penetrated the ravines which lava to judge from. The exceedingly compact axe-stone, how- 

 intersect the northeastern slope of Mauna Kea, so far as to ever, is found in several places near the summit of the moun- 

 examine the solid core of the mountain. As the summit pre- tain. 

 sents no crater walls, we have only the surface overflows of 



