﻿Geology and Mineralogy. 235 



occasioning " the form due to a self-supporting mass of coherent 

 material." Other causes mentioned by Mr. Milne are the ten- 

 dency in the larger particles to roll farthest down the mountain ; 

 atmospheric denudation ; position of crater ; direction of ejection ; 

 direction of wind during an eruption. These are all conditions 

 pertaining to cones consisting chiefly of fragmental material and 

 have little bearing on the larger volcanic cones or mountains 

 consisting mostly of lavas, among which are those of Hawaii. 

 Outflowing lavas, by cooling around the central vent, with adhe- 

 sion between the successive outflows, may, as Prevost long since 

 stated (and illustrated by means of beeswax), make slopes of all 

 angles from 1° to 90°. The fusibility of the lavas, the degree of 

 heat above that of fusion, the size of the outflows, the position of 

 the Assures and outflows, all have their effects. Orthoclase lavas, 

 owing to high fusing point, make steep cones or domes, and 

 basaltic lavas those of gentle slope, sometimes as low as 3°. 

 Mount Loa, one of the latter kind, the angle 3° to 8°, has a double 

 curvature in its surface — a flattened or dome-shaped top and wide 

 spreading nearly flat base on its south and southeast sides.* But 

 although almost 14,000 feet high, it is not near the crushing limit 

 of height ; for, as Mr. Milne states, even sandstone could be built 

 up, on a base of nine miles in diameter, to a height of 14,500 feet 

 without crushing, and granite to a height of 20,000 feet. The 

 base of Mount Loa is from forty to fifty miles in diameter, and 

 virtually larger, because braced on the north and west by Kea 

 and Hualalai, and the slope about the top is very small. All vol- 

 canoes over 5,000 feet in height are probably made at least one- 

 half of solid lavas, although having, it may be, an exterior of 

 fragmental material. The problem of form is therefore a com- 

 plex one. J. d. d. 



2. Summit crater of Mt. Loa, Mokuaweoweo. — Mr. Alexaist- 

 dee, Surveyor General of the Sandwich Islands, has surveyed the 

 summit crater of Mt. Loa, and we have from him a photograph 

 of his map and an account of the survey. He makes the highest 

 point, on the west margin of the pit, 13,675 feet above the sea- 

 level. As mapped, also by Wilkes, it has a large central pit, with a 

 smaller one on both its northern and southern sides, each of 

 which communicates with the central pit. The length of the 

 whole is 19,500 feet; the greatest breadth 9200 feet; the great- 

 est depth 800 feet; area of central crater 2*3 square miles. It 

 is a marvelous fact that a volcano of so great height should 

 be so frequently in eruption — not usually over its brim, but 

 through fissures commonly 1000 to 3000 feet below it. But, 

 notwithstanding the depth of the pit, outflows from fissures 

 opening to the very summit, and directly on its margin, some- 



*The writer's Manual of Geology, on page 724, has a profile of the island of 

 Hawaii, as seen from the eastward, copied from his Exploring Expedition Geo- 

 logical Report (p. 159), which was drawn by him with much care when at sea off 

 the island; it shows, besides the outline of Loa, the contrast between it and that 

 of Mt. Kea. In the view the position of Kilauea was not distinguishable. 



