THE EXTRUSIVE PROCESSES. 615 



western America which seems to hold fairly well in its general aspects, 

 though not everywhere completely realized, so far as surface observa- 

 tion goes. Richthofen's order is: (1) lavas of neutral types, (2) lavas 

 of acid types, (3) lavas of basic types, (4) lavas of more acid types, and 

 (5) lavas of more basic types. The special varieties of rock vary, and 

 even the general order is often apparently defective. The defects are 

 sometimes assigned to the concealment of some of the outflows. While 

 this may be true in some cases, it is not unlikely that in others there is 

 a real failure of the sequence. At any rate, the sequence can only be 

 regarded as a rough generalization. It is supposed to be due to mag- 

 matic differentiation caused by the differences of temperature to which 

 the different parts are subjected underground, by differences of specific 

 gravity and fluidity which result from changes of temperature, and 

 probably by other causes. 



Temperatures of lavas. — Accurate determinations of the tem- 

 peratures in the center of the lava-columns, where they have been 

 least reduced by contact with the rock-walls, have not yet been 

 made, but it is clear from the whiteness of the lavas that their 

 temperatures are often appreciably above the melting-point. This is 

 also a necessary inference from the length of time they remain fluid, 

 notwithstanding the great surface contact of the column in its miles 

 of ascent, the conversion of contact water into steam, and the ex- 

 pansion and escape of the gases. In cases where determination has 

 been practicable (and they certainly do not represent the maximum 

 temperatures) it has been found that the melting-points of silver, 

 about 960° C, and of copper, about 1060° C, are reached. In con- 

 nection with overflows, it has been found that brass is decomposed 

 into its component metals, the copper actually crystallizing. Silver 

 has been sublimed, and made to redeposit itself in crystalline form. 

 This implies much more than the bare melting temperatures. Even 

 the fine edges of flints have been fused. It is, therefore, probably safe 

 to assume that the original temperatures of the lavas as they rise to 

 the surface sometimes reach considerably beyond 2000° Fahr. (1093° C), 

 and may perhaps even attain 3000° Fahr. or more. Even these 

 temperatures must be somewhat below the original subterranean 

 temperatures of the lavas, because some heat must necessarily be 

 lost in rising, partly by contact with the walls of the colder rocks 

 through which they pass, probably for as much as a score of miles at 



