BASALT VOLCANOES.
Volcanoes may be classified by the petrographical character of the lavas which they emit : for the fluidity of the material, the tem perature of fusion and the shapes of the cones are determined by the assemblage of the constituent minerals. The materials are spoken of as basic or acidic. The basic lavas are those carrying iron, lime and magnesia in large amount, and small proportions of silica or the lime-soda feldspars, the augitic and allied ferro magnesian silicates, iron-oxide, but no free quartz. The rocks thus made up are termed basalts, dense and fine grained, with or without olivine. They arc the most fusible of the lavas, melting at the temperature of 2250° F. The acid lavas carry much silica, potash feldspars, hornblende, mica and quartz and are termed trach','tic and rhyolitic. They fuse with difficulty from 2700° F. to 3100° F. Between these two classes are intermediate forms, composed of lime soda feldspar, augite or hornblende and f re quently quartz, and have a medium degree of fusibilty, 2520° F.
The form of the cone is dependent upon the degree of fusibility. Those of the basalt order, easily liquified, build up cones with slopes of less than ten degrees ; those of the trachytic order build up cones with slopes from twenty-eight to thirty-five or forty degrees. It is the degree of fluidity or pastiness of the lava that produces the shapes of volcanoes, modified by the presence of tuff or cinders.
With these distinctions in mind it is easy to perceive that most of the Hawaiian volcanoes belong to the first order as they are basaltic and have low angles of slope. Mauna Loa is the finest example, being a magnificent dome, concave above where the basalts have accumulated, but with a concave rim at the base, be cause of eruptions low down. One may be at a loss within a mile of the summit to know where the pit is. That was my experi ence in 1883 when my guide lost his way because the marks upon which he had relied for his knowledge of locality were covered with snow. The surface is nearly flat for several square miles
in extent, and the pit is not seen till you are close upon it. The form of the dome of Mauna Loa is shown in Plate 15, as seen from a distance of twenty miles. In a circle of five miles around the pit the mean slope is about three degrees. Radial lines to the different points of the compass show slopes from about four to six and two-thirds degrees from the summit to the shore line. The slope of the cone of Halemaumau inside of Kilauea is not much more than a single degree. The outward slopes of Kilauea are about one and a half degrees to the northeast and southwest. The south slope is steeper because of displacement by faulting.
These domes are elongated, being elliptical rather than circu lar. Mokuaweoweo has the course of N. 35° E. for the northern part and N. 20° E. for the southern part, as shown upon the map. About the same trend prevails from the flows near Kahuku to the summit, and beyond to Mauna Kea. To reach the sources of the flows in the south edge of Hamakua connected by the fault line, the direction is about northeast. This southern divergence is paralleled by the similar bend southeast from Kilauea.
Variations in the character of the lava are conceivable, as of rhyolite in the place of basalt ; in which case the features of the cone might be altered. Or the supply of heat may be diminished, and allow the outburst of cinders and tuffs : or should water enter the conduit from below, explosive eruptions might occur, like those of the other types of action. The tuffs and ashes emitted explosively upon the other islands seem to be connected with the later lateral monticles.
The most important feature of the basalt volcano is the cal dera or pit crater, and this is thoroughly characteristic of Hawaii. The eruptions are always within or from rents connected with the central body of lava. This feature dominates thoughout the archipelago, and there are nowhere else upon the planet finer ex amples of this type of action.