2S B. A. Daly — Mechanics of Igneous Intrusion. 



expands- in the interval 20°-1300° C. as much as 0-000025 vol- 

 ume per degree Centigrade. (Barns and Reade — see first paper). 

 This average may safely be employed as a means of determin- 

 ing the minimum decrease of density which each rock-type 

 undergoes in passing into the molten condition. On this basis 

 the writer has constructed the preceding table (II), which shows 

 the changes of specific gravity at convenient temperature 

 intervals. 



Table III shows the changes in specific gravity undergone 

 by blocks of stratified and schistose rocks (common country- 

 rocks about batholiths), as these blocks assume the tempera- 

 ture (1300° C.) of molten magma in which they are immersed. 



Table III. 



Eange of sp. gr. Kange of sp. gr. 



at 20°C. at 1300°C. (solid) 



Gneiss 2-60-2-80 2'52-2-7l 



Mica schists ... . 2-75-3-10 2-67-3-00 



Sandstone 2-20-2-75 2-13-2-67 



Argillites 2-40-2-80 2*32-2-71 



Limestone 2-65-2-80 2-57-2*71 



It appears from these tables that nearly all xenoliths must 

 sink in any molten granite or syenite ; most xenoliths must 

 sink in molten quartz-diorite, tonalite or acid gabbro. Many 

 xenoliths might float on basic gabbro but the heavier schists 

 and gneisses must sink in even very dense gabbro magmas at 

 1300° C. 



Giving, then, the highest permissible values to the specific 

 gravities of magmas, it is still true that blocks, such as are 

 shattered from the wall or roof of a batholith, must sink when 

 immersed in most magmas at atmospheric pressure. As shown 

 in the first intrusion paper, the blocks would likewise sink, 

 though the magma enveloping them lies at depths of ten or 

 fifteen kilometers below the earth's surface. 



Sinking of the shattered blocks — It has been objected to the 

 stoping hypothesis that the viscosity of granitic magmas is too 

 great to allow of the sinking of blocks even much denser than 

 those magmas.* This objection has, however, never been 

 sustained by definite experimental or field proofs. The xeno- 

 liths visible along batholithic contacts have assuredly not sunk 

 far from their former positions in wall or roof and the reason 

 for this must be sought in the high viscosity of the magma. 

 High viscosity is an essential attribute of a nearly frozen 

 magma. The phenomena of fractional crystallization and of 

 magmatic differentiation unquestionably show that each 



* Cf . W. Cross, G. F. Becker, and A. L. Day, Science, xxv, p. 620, 1907. 



