740 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V., A. 1912 



Humphrey.* He subjected panels composed of different dressed stones to rapidly 

 and steadily increasing furnace-heat. After periods of but ten to forty minutes 

 many of the stones spalled to depths of one to two inches, and all the blocks 

 were badly cracked. Quenching with cold water or with draughts of cold air 

 naturally increased both the spalling and cracking. The experiments show 

 that quenching by water or cold air is not the necessary condition for the yet 

 more remarkable spalling of stone in city fires. 



It may be noted that the shattering of crystals and rock-fragments, when 

 immersed in silicate melts, has often been observed.^ The strains are, in such 

 eases, necessarily of a lower order than those developed on the wall of a batho- 

 lith where, therefore, shattering is even more certainly brought about. (See 

 Plate 33). 



Finally, the disruptive power of volatile matter contained in the wall-rock 

 heated by batholithic magma, should be considered. This power may be very 

 great. 



In view of all the facts there seems to be a sheer necessity for believing in 

 contact-shattering through differential heating and expansion in the thin shell 

 o£ a country-rock which encloses a large body of molten magma. The evidence 

 for the shattering is often exceedingly full and clear in the field. The broad 

 or narrow belts of xenoliths so often found just inside the main contacts of 

 batholiths are very hard to explain if those batholiths are due to laccolithic 

 injection. The blocks are characteristically angular; they are generally not 

 arranged with their longer axes parallel, as if they had been pulled off from the 

 vails by the friction of the moving magma. On the laccolithic theory one 

 would expect many of the xenoliths to form elongated smears in the granite 

 rock. This is indeed occasionally seen but most exceptionally; as a rule the 



xenoliths have just that irregularity of form and arrangement which they should 

 have if they had been shattered off by the hot magma just before its final con- 

 solidation. Throughout its long, earlier history the magma must, in every case, 

 have had a much more effective shattering power. 



Relative Densities of Magma and Xenolith. — In his first intrusion-paper, 

 rbe writer published the results of his attempt to calculate the possible specific 

 gravities of the chief types of molten magmas under plutonic conditions. The 

 calculations were based on Barus's well-known fusion experiments on diabase. 

 The specimen investigated had a specific gravity of 3-0178; when fused to a 

 glass and cooled to 20° C, a specific gravity of 2-717. He further states that 

 the glass:}: showed an expansion of 3-9 per cent in 'melting' and, as glass, 

 expanded 0-000025 in volume for a temperature rise of 1° C. through the interval 

 0°-1000° C. and 0-000047 in volume for 1° C. through the interval 1100°- 

 1500°. The 'melting' expansion (solidification-contraction) and the varying 

 rate of expansion (or contraction) above and below 1000° C. seem to show that 



• R. L. Humphrey, ' The Fire-resistive Properties of various Building Materials/ 

 Bull. 370, U.S. Geol. Survey, 1909. See especially page 69 and plate 31. 



t Cf. C. Doelter and E. Hussak. Neues Jahrb. fur Min. etc., 1884, p. 18; A. Becker, 

 Zeitschr. d. d. geol. Ges., Vol. 33, 1881, p. 62. 



J ' Throughout this paper the molten rock solidifies into an obsidian.' C. Barns in 

 Bull. 103, U.S. Geol. Survey, 1893, p. 26. 



