



744 DYNAMICAL GEOLOGY. 



The leucite of the Vesuvian lavas is an infusible mineral; and these lavas, near the 

 surface, are in a half-fused or pasty state, having unusual viscidity. Experiments at 

 Vesuvius have obtained for the flowing lava streams a temperature above the fusing 

 point of copper as well as silver. Davy found that a copper wire one twentieth of an 

 inch in diameter, and a silver wire of one thirtieth, when thrust into the flowing lava at 

 Vesuvius, near its source, instantly melted. The copper indicates a temperature of 

 2426 D F. ; and this may be much less than that which exists a yard below the surface. 

 At what depth in the volcano the leucite takes its solid form is undetermined. The 

 crystals sometimes contain grains or crystals of augite, showing that the auyite crystal- 

 lized first; and, also, that the temperature at which they are formed may be no greater 

 than that required for the augite. The mineral may come from the alteration of ortho- 

 clase in rocks adjoining the seat of fire, as it most resembles this species in constitution. 



The view that the fusion of lavas is due to the combined action of moisture and heat, 

 or is aqueo-igneous, was early presented by Scrope, and has been held by later writers. 

 But while the steam present in them increases their mobility, it does not appear, in view 

 of the above-mentioned facts, to be essential to their fusion and flow. 



With decrease of temperature lavas may increase in viscidity in three 

 ways : (1.) By a close approach to the limit of fusion, as in ordinary 

 glass ; (2.) by incipient crystallization of the ingredients, a pasty con- 

 dition remaining because a portion is still in fusion, as happens with 

 molten cast-iron ; (3.) by the less fusible portion crystallizing, or be- 

 coming individualized, leaving a more fusible part to keep up a slug- 

 gish flow. When there is a slow decline in the activity or heat of 

 a volcano, the overflows for a long period may generally consist of 

 lavas that have reached a thickened or pasty state in one of these 

 ways. Masses of lava thrown from the stream at the eruption of 

 Kilauea, in 1840, caught around the branches of trees and clasped 

 them, and with only a scorching of the bark, showing that though 

 plastic they were previously half solidified. Ortboclase lavas pass 

 most readily into this sluggish state because of their less fusibility, 

 and sometimes make dome-like hills from the swelling up of the thick- 

 ened, feebly mobile material, as in the extinct volcanic region of 

 Auvergne. 



The semi-glassy lavas called rhyolytes, often show that the constituent minerals of 

 the material become crystallized or individualized together even when different in 

 fusibility. Thus in obsidians belonites and trichites appear together, as the first step 

 toward lapiditication ; and, in a later stage, orthoclase and hornblende or augite exist 

 together in the glassy base, with sometimes also a triclinic feldspar ; and these increase, 

 relatively to the amount of glass, as the passage to stone becomes more and more com- 

 plete. 



2. Conditions of volcanic and non-volcanic igneous action. — 

 (1.) Volcanic Action. — This subject may be best illustrated by the 

 facts afforded by the great Hawaiian volcanoes. The map on page 

 725 shows the positions of the two active craters, 16 miles apart ; and 

 the following section, the relation between the sizes of the craters and 

 the height of the mountain. It is to be kept in mind that the lavas, 

 previous to several eruptions of the summit-crater, stood about 13,000 



