REPORT OF THE CHIEF ASTRONOMER 755 



SESSIONAL PAPER No. 25a 



xeuolith 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° C.,* 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 undercooling), therefore, magmatic stoping is still possible. The lowest 

 limit of active assimilation cannot well be much below 1000° C, while the tem- 

 perature 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 confined, within the 

 interval 120O°-'3OO° 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 aggregating 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 batholith 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 favour of the stoping hypothesis. 



Abyssal Assimilation. — In the first paper on the stoping hypothesis the 

 writer stated grounds on which one must believe in the complete solution of 

 engulfed xenoliths. One has only to imagine a block of gneiss, say ten metres 

 in diameter, sinking through a column of superheated basalt twenty or thirty 

 kilometres deep, to become convinced of the ultimate fate of that block. If 

 the somewhat cooled lavas described by Lacroix.f von John,:}; Dannenberg,§ 

 Sandberger*"* and others could dissolve rock-inclusions in the notable way des- 

 cribed by those authors, we must credit a vast solutional efficiency to plutonic 

 magma when it attacks similar blocks in great depth. The lava has a few 

 hours or days in which to do its work; the abyssal magma has centuries if not 

 a large part of a geological period! 



It must be remembered that geosynclinal sediments are rocks unusually 

 rich in water, chlorides, sulphur trioxide, etc. ; all substances aiding solution in 

 the primary magma and in the secondary (syntectic) magma itself. It is 

 probably also owing to these fluids in large part that granitic magmas have 

 crystallized at comparatively low temperatures. . „ 



The conception of stoping with abyssal assimilation has many more points 



* Cf . A. L. Day and E. S. Shepherd, Jour. Amer. Chem. Soc, Vol. 28, 1906, p. 1099. 

 t Les Enclaves des Roches Volcaniques Macon, 1893. 

 X Jahrb, d. k. k. Reichsanstalt, Vienna, Vol. 52, 1902, p. 141. 

 § Tscherm, Min. u. Petrogr. Mitth., Vol. 14, 1895, p. 17. 

 ** Sitzungsber. K. Bair. Akad. Wiss., 1872, p. 172. 

 25a — vol. iii — 49 



