THE WILKES PARTY UPON MAUNA LOA.
The most elaborate attempt to take observations upon Mauna Loa was that of the United States exploring expedition in 184o-41. Captain Wilkes, the officer in command of the expedition, wished to apply the best apparatus of his time for the determination of geodetic positions and altitudes besides observing the volcanic phenomena and mapping the country. His ship anchored at Hilo. The party started December 14, 1840, and the last of them returned to Hilo, Jan. 23, 1841, making an absence of forty two days. Twenty-eight days were spent upon Mauna Loa ; six days were required to make the ascent and two for the descent to Kilauea. At the beginning the company was to be compared to a caravan. It consisted of two hundred bearers of burdens, forty hogs, a bullock and bullock hunter, fifty bearers of poi, twenty five with calabashes of different shapes and sizes, from six inches to two feet in diameter. Some of the bearers carried the scientific apparatus, others parts of the house to be erected on the summit, tents, knapsacks and culinary utensils. There were lame horses and as many hangers on as there were laborers. The natives moved under the direction of Dr. G. P. Judd, without whose help the expedition would have been a failure. After the start thirty more natives were added to the company so as to equalize the bur dens. After passing Kilauea the number of the party was somewhat reduced, but there were still three hundred persons in all to be provided with food and water. Sickness and accidents led to the establishment of the Recruiting Station or hospital at the altitude of 9,745 feet. All the party experienced more or less of mountain sickness. The final encampment was on the edge of the pit of Mokuaweoweo, and the party suffered much from the inclement weather. There were a dozen separate tents and houses, all surrounded by a high stone wall. These are shown in Plate 16A. Fifty men were detailed from the vessel to complete the undertaking. The serviceable natives returned down the mountain after the necessary articles had been brought up, and came back after the termination of the observations in order to transport this valuable apparatus back to the ship.
The following facts were stated about the mountain : Its whole area was of lava, chiefly of very ancient date, rough and seemingly indestructible, made up of streams that had flowed from the central vents for many ages. Both pahoehoe and clinkers (aa) abounded. Wilkes concluded that the clinkers were formed in the great pit where they were broken and after wards ejected with the more fluid material. Their progress would have continued till the increased bulk and attendant fric tion arrested the stream. Pahoehoe seemed to have flowed from the clinker masses that had been stranded. The crater was likened to an immense caldron, boiling over the rim, and dis charging the molten mass and scoria which had floated on its top.
From the plan of Mokuaweoweo as given by Wilkes, Plate 17A, the following points may be made. The central part is the
deepest, seven hundred and eighty-four feet by the west bank and four hundred and seventy feet by the east. This part is 9,00o feet in diameter nearly circular. The bottom is flat, with ridges from ten to fifty feet high, alternating with deep chasms and pahoehoe. Skirting this pit on both the north and south sides are lunate platforms apparently two-thirds as high as the summit rim, both together having an area perhaps half that of the main depression, and their outer rims coincide with the out line of the whole caldera. Just outside of both are smaller pits, the northern one two hundred feet and the southern nearly three hundred feet in diameter. The last has the name of Pohaku o Hanalei from Wilkes, showing seventy layers of basalt in the walls, and a cooled stream of lava that came from the larger crater. A smaller pit-crater is mapped to the south. There are many deep fissures about these pits and the lava has a very fresh appearance, being suggestive of obsidian. From the Pohaku o Hanalei a great steam crack points southerly. The highest point in the rim is opposite the encampment, with the altitude of 13,780 feet, three hundred and forty feet higher than at the station, which had the name of Pendulum Peak. Mauna Kea proved to be one hundred and ninety-three feet higher than Mauna Loa. Water boiled at 187° Fah. at Pendulum Peak. For some reason the main axis of Mokuaweoweo was placed at N. and S. instead of N. 26° E. It differed from Kilauea in the absence of a black ledge and a boiling lake and the evidences of heat were scant. There was one cinder cone at least upon the floor. Sodium and calcium sulphates, magnesium and calcium carbonates, am monium sulphates and sulphurous gases were met with in the pit.
The clinkers were compared to the scoriae from a foundry, in size from one to ten feet square, armed on all sides with sharp points. The fragments are loose with a considerable quantity of the vitreous lava mixed with them.
As to origin, both the smooth and rough varieties are conceived to have been ejected in a fluid state from the terminal (summit) crater. The "clinkers" are seldom found in heaps, but lie ex tended in beds for miles in length, sometimes a mile wide, and occasionally raised from ten to twenty feet above the general slope of the mountain. The "clinkers" were formed in the crater itself, broken up by contending forces, ejected with the more fluid lava, which carried it down the mountain slope until ar rested by the accumulating weight or by the excessive friction. They were streams of lava : and this opinion was fortified by the observation that pahoehoe came out from underneath the masses of clinkers wherever they had stopped. The crater was an im mense caldron boiling over the rim. No facts are presented in favor of this view, and the idea was evidently borrowed from the conception of what a volcano should be. There had been no signal eruption previous to 1840 when the characteristic stream flows of this mountain had been developed.