VOLCANOES. 743 



3. Heat of Lavas and Conditions of Igneous Action. 



1. Temperature of Fusion. — The temperature of the lavas of large 

 free-flowing craters in full action is often not less than 2,200° F. with- 

 in 2 or 3 feet of the surface ; and the feldspathic and augitic (or horn- 

 blendic) portions are in complete fusion. 



The chrysolite and magnetite, present in much of the rock, are 

 infusible minerals, and hence may be solid grains floating in the lava. 

 The Eifel bombs are evidence that this was the condition in the vent 

 whence they were ejected. But their condition below is uncertain. 



(or.) The above statement as to the degree of heat is proved by facts at Kilauea. 

 Over the great lake of lavas in the pit, in 1840, the play of jets brought into view the 

 hotter lavas beneath them, and produced a brilliant spangling of white light. The jets 

 were but 60 to 300 feet in height, and hence the white-hot lavas beneath, brought into 

 view by the movements, were quite near the surface. This indicates a temperature 

 of at least 2200 D F. The heat is sufficient not only to retain the lavas in a melted state, 

 but, should they become thickly crusted over, to remelt the hard crust (after first break- 

 ing and submerging it) and make it join in the boiling. 



(b.) The fusion of the labradorite and augite, the chief constituents of the lava, was 

 complete. The volcanic glass of Kilauea has the composition of ordinary doleryte. The 

 mean of two careful analyses of the capillary variet}"- (called Pole's Hair), made in 

 1879 by Mr. T. J. Allen, gives Silica 5075, A1 2 3 16-53, Fe 2 3 2-10, FeO 7-89, MnO trace, 

 Mg0 7-65, CaO 11-96, Na 2 2-13, K,0 0-56, ignition 0-35 = 99-92; which is almost 

 identical with the composition of the Mesozoic doleryte of the Connecticut valley. This 

 glass hence contains the feldspathic and augitic ingredients in complete fusion, and at 

 a low temperature compared with that of the mass of lava; for the glass is from the 

 superficial scum of the lava pool, the fibres being made by the transporting winds car- 

 rying off points from the lava jets. 



Again: Plattner found, by comparing with the fusibility of alloys of platinum and 

 gold, the fusing temperature of slags from iron furnaces to be 2130° F. to 2452° F. ; and 

 Mehrbach obtained, in a similar way, for an iron furnace slag containing 37 to 40 per 

 cent, of iron protoxide (one of them near an iron-augite) 2400° F. to 2480 3 F. ; and for 

 another, near a scapolite, 2608° F. (Mehrbach's Anw. der Geblaseluft, Leipzig, 1840). 

 These temperatures are to be taken as only approximations; but, with the largest allow- 

 ance for probable error they sustain the above statements. [2016° F. was taken for the 

 fusing point of gold, and 4593° F. for that of platinum.] 



Further, labradorite and the ordinary augite of volcanoes have about the same easy 

 fusibility, each being marked 3 on Von Kobell's scale; and the latter has often been 

 obtained in crystals from furnace slags. 



There is hence no reason for questioning the complete fusion of this part of the lavas. 

 The fusibility of native silver is marked 2 to 2*5 by Von Kobell, and is 1904° F. accord- 

 ing to determinations at the Utrecht mint in 1869. 



(c.) The material of the soda-feldspars (albite, oligoclase) and potash feldspars (or- 

 thoclase, microcline), is also in complete fusion in many lavas. Most obsidian is essen- 

 tially orthoclase in a glassy state; and at Lipari there are streams of this glass, with 

 lines showing a sluggish flowing movement; and thus the above statement is true as 

 far as orthoclase or its ingredients are concerned. Now orthoclase is the least fusible 

 of the feldspars, being marked 5 on Von Kobell's scale (and microcline is the same); 

 while albite is marked 4, and oligoclase 3*5. In an ordinary oven for baking porcelain, 

 at Trenton, in New Jersey, albite (from Branchville, Conn.) was easily fused down, 

 while the microcline of the same locality was only fused over the surface. The temper- 

 ature of fusion of ordinary glass is not easy to obtain because it is plastic (or undergoes 

 " vitreous fusion," as it is called) long before the fusion is complete. 



