and Bock Densities at High Temperatures. 3 



large, while on the other hand the process of fusion frequently 

 changes the original composition by driving off water or other 

 volatile constituents, a change which may lead to an apparently 

 smaller volume for the glass than for the crystal. 



There are a few cases known of natural glasses which are 

 denser than the corresponding rocks. It is not altogether cer- 

 tain, however, that this is not due to small differences in com- 

 position. Thus, Delesse* found that the glassy portion of 

 certain basic dikes in Scotland was from 1*5 to 4 per cent 

 denser than the crystalline portion. 



In general, it may be said that if the glass has a considerably 

 greater volume than the crystalline solid (10 per cent or more), 

 it is fairly certain that the solid will expand in liquefying. 

 But as a basis for comparing the volume at high temperatures 

 of one type of rock in the crystalline state with another type 

 of rock in the molten state, the volumes at ordinary tempera- 

 tures are quite unreliable. 



Evidence that is even more indirect than the comparison of 

 volumes at ordinary temperature has been offered by other 

 authors. Some of these have supported a supposed general 

 expansion of rocks in crystallizing. For instance, Langf in 

 1875 concluded, from microscopic observation of basalt in com- 

 parison with gypsum, that jointing of rocks is due not to con- 

 traction but to the pressure developed by expansion during 

 solidification. Stiibel^: has based a theory of volcanicity upon 

 such a supposed expansion. This same theory was put forth 

 by Bornemann in 1887.§ He found experimentally that mel- 

 anite crystals inclosed in leucite can be fused without disturbing 

 the leucite, and that the fused silicate cools to a glass containing 

 bubbles or vacuoles, indicating that the glass occupies a smaller 

 volume thau the original crystal. | The amount of the contrac- 

 tion was estimated to be 4 per cent. 



On the other hand, Gilbert, in 1877,1" estimated that the tra- 

 chyte of the Henry Mountains had contracted by about one- 

 tenth of its volume from the liquid to the cold crystalline state. 

 This estimate was based on the densities of the sedimentaries 

 and on the hypothesis that the horizon of the laccolitic intru- 

 sions was determined solely by the relative densities of the 

 liquid magma and the invaded rocks. 



*Delesse, Ann. Mines (5). xiii, 369, 1858. 



f Jahresh. d. Ver. f. Vaterland. Naturk. in Wiirttemburg, xxxi, 336, 1875. 



% A. Stiibel: Die Vulkanberge von Ecuador, p. 367-376, Berlin, 1897. 



§ J. C. Bornemann: Ueber Schlackenkegel und Laven. Koenigl. Preuss. 

 geol. Landesanst, Jabrb., 279, 1887. 



1 Ibid., p. 252-258. 



if Geology of tbe Henry Mountains, p. 72-80. (U. S. Geographical and 

 Geological Survey Eocky Mt. Region, 1877.) 



