38 B. A. Daly — Mechanics of Igneous Intrusion. 



masons by batholithic magmas consists in emphasizing the 

 obvious fact that the average xenolith and average wall-rock 

 of batholiths do not show direct evidence of melting or of 

 solution in the granitic magma. This objection has been 

 answered by the writer in several publications* and also by 

 Andrews in most vivid fashion. f The point has, however, 

 been restated by several authorities without any adequate dis- 

 cussion of the subject. No one can deny that, when the magma 

 is all but frozen, it is incapable of assimilating xenolith or 

 wall-rock on any large scale. The practical question is as to 

 the magma's efficiency during the long antecedent period of its 

 history. It is true that bed-ridden centenarians did not build 

 the pyramid of Cheops ; it does not follow that men did not 

 build it. 



If it be assumed that the quartz of granite has crystallized 

 at or below 800° G.,J it follows that complete rigidity is not 

 established in a granite batholith until it has cooled to at least 

 800° C. Down to about that temperature limit (of undercool- 

 ing), therefore, magmatic stoping is still possible. The lowest 

 limit of active assimilation cannot well be much below 1000° C, 

 while the temperature required to melt the average xenolith 

 is about 1200° C. As the viscosity of granitic magmas increases 

 greatly below 1200° C, diffusion and convection must become 

 rapidly inadequate to remove syntectic films at main contacts, 

 so that the molecular lowering of the fusion-point will be con- 

 fined, within the interval 1200°-800° C, chiefly to the sunken 

 blocks. It follows, first, that in the very long period of time 

 occupied in the cooling of a plutonic mass from 1200° C. to 

 800° C, there will be little or no melting or solution of wall- 

 rock ; secondly, that many shells of roof-rock, perhaps aggre- 

 gating thousands of feet in thickness, may be stoped away 

 during that same period of time. In other words, because the 

 shatter-period is longer than the period of active assimilation 

 at the roof, it is an essential feature of the stoping hypothesis 

 that neither visible xenolith nor main wall of a granite batho- 

 lith should normally show a collar of assimilation. So far 

 from being a difficulty, the fact that this is generally true is a 

 distinct argument in favor of the stoping hypothesis. 



Abyssal assimilation. — In the first paper of this series the 

 writer stated grounds on which one must believe in the com- 

 plete solution of engulfed xenoliths. One has only to imagine 

 a block of gneiss, say ten meters in diameter, sinking through 

 a column of superheated basalt twenty or thirty kilometers 



*This Journal, xv, p. 281, 1903; Bull. Geol. Soc. of America, xvii, p. 

 372, 1906. 



f Records, Geol. Surv. of N. S. Wales, viii, Pt. 1, p. 126, 1905. 



% Cf . A. L. Day and E. S. Shepherd, Jour. Amer Chem. Soc, xxviii, p. 

 1099, 1906. 



