and Sock Densities at High Temperatures. 39 



conditions) irreversible dilatation due to the formation of 

 cristobalite. 



Granite has a curve of similar form. Above 575° granite 

 undergoes a permanent dilatation caused by the shattering 

 effect due to the different expansion coefficients of its minerals 

 and to the escape of gases. 



The volume of diabase glass was determined up to 1250° 

 and compared with Barus' well-known values. A 6imple 

 explanation of some of the conflicting features of his observations 

 has been found. 



Diabase expands considerably in liquefying. The difference 

 in volume between crystalline and glassy (liquid) diabase at 

 1150°, at which temperature the crystalline rock begins to fuse, 

 is about 109 per cent of the volume of the glass, or 12 per cent 

 of the volume of the crystalline rock at that temperature. At 

 0° this difference is 7 - 7 per cent of the volume of the rock. 



A consideration of the comparative specific volumes of 

 quartzite or feldspathic sandstone on the one hand, and liquid 

 diabase at the same temperature on the other hand, shows that 

 the diabase is denser, and slabs or fragments of the sandstones 

 would therefore tend to float in it. This seems the best explana- 

 tion of the peculiar sandstone inclusions in the Palisade diabase 

 described by Lewis. 



Volatile components, now absent from the diabase as we 

 find it, seem to have greatly modified its temperature of 

 intrusion, and make it impossible at present to solve completely 

 the problem of relative volumes under the original conditions 

 of intrusion. The foregoing measurements, however, furnish a 

 necessary basis, on which the variation due to volatile components 

 can be established. 



Natural rocks are unsatisfactory for measurements of this 

 kind for three reasons : (1) from the chemical standpoint they 

 are very complex systems, between whose properties it is 

 difficult to find a simple relation : (2) the unequal expansion of 

 the various minerals fractures the rock so that it is impossible 

 to determine its true volume change over any considerable 

 range of temperature ; (3) the natural rocks give off considerable 

 quantities of gas. The best plan for future work, therefore, 

 would seem to be to determine the volumes at high temperatures 

 of simple silicates, prepared in the laboratory and free from 

 gases. Their other properties can then be controlled, and it 

 will become possible to find definite relationships between 

 composition and dilatation. Such measurements have been 

 begun, and will be reported on at a later date. 



Geophysical Laboratory, October, 1913. 



