R. A. Daly — Mechanics of Igneous Intrusion. 281 



to 1150° C. for granite, from 1070° to 1010° C. for phonolite, 

 and from 1060° to 992° C. for basalt.* It is clear from 

 Barus's researches that similar differences would be expected 

 under plutonic conditions of pressure. "Without dwelling on 

 the older experiments proving that the presence of water and 

 " mineralizers " aids in giving a magma mobility, the more 

 recent one by Barus may be noted. f He has been able to fuse 

 glass at 200° C. in the presence of water! Finally, Oetling 

 found that, when a magma is once molten, the pressure of 200 

 or 300 atms. tends to keep it molten longer than under normal 

 pressure.^ Amagat, before him, came to the view that, pro- 

 vided the pressure be high enough, solidification may be indefi- 

 nitely held back.^ However this may be, experiment fully 

 substantiates the conclusion enforced by field evidences, that 

 plutonic magmas are highly fluid during active intrusion.! 



Enclosures of foreign rocks in the endomorphic zones of 

 intrusives. — To return to the original problem of the country 

 rock inclusions actually seen in the field at internal intrusive 

 contacts. We have found grounds for believing that such 

 blocks could .not, in the vast majority of occurrences, have 

 remained in their present positions close beside the niches in 

 .the wall or roof whence they came, unless the magma enclosing 

 them were very viscous at the time they were rifted off. 

 Additional testimony to this mechanical indication of high 

 viscosity is derivable from the normal chemical inactivity of 

 the magma in contact belts of the kind. The blocks, as a rule, 

 show few absorption phenomena or none at all, and are but 

 rarely accompanied by diffusion aureoles. The boundaries of 

 the fragments against the igneous rock are more often than 

 not exceedingly sharp, like the new molar-contact surface 

 formed by the rifting away of the same blocks. Lastly, the 

 contact-metamorphic changes wrought in the blocks are often 

 no greater than those effected in the country rock many yards 

 from the molar contact. Viscosity of about the same degree 

 will explain the suspension of the basic segregations in granites, 

 syenites, etc., and probably approaches that of the Archaean 

 granitic magmas which, according to Lawson, were capable, 

 under enormous dynamic stresses, of shearing and attenuating 

 foreign blocks suspended in those magmas near the moment of 

 consolidation of the latter. Lawson has also suggested that, 

 although the viscosity was so great, the temperatures may have 

 been high enough to melt up the more basic fragments com- 



*Tscher. Min. u. Petrog. Mitth., xx, 232 (1901). 

 + This Journal, vi, 270 (1898). 

 llTscher. Mm. u. Petrog. Mitth.. xvii, 332 (189T). 

 gCoinptes Rendus, No. 16 (1893). 



|| Cf. Iddings. Jour. Geol.. i. 833 (1893); A. Geikie, Ancient Volcanoes of 

 Great Britain, ii, pp. 413-22-39 (1897). 



