280 It. A. Daly — Mechanics of Igneous Intrusion. 



3. Blocks of the heavier granites and syenites would sink in 

 all magmas except the more basic gabbros and peridotites. 



4. Blocks of all the average sediments and schists would 

 sink in any molten magma except the denser gabbros and 

 peridotites. 



5. Blocks of many of the sandstones would float in syenitic 

 (trachytic) magma, and argillites would float in the denser kinds 

 of the same magma. 



6. The lighter gneisses, argillites, quartz rocks, and probably 

 many limestones, would float in the average gabbro magma, in 

 which, however, the other sediments and schists would sink. 

 This means that the great majority of the solid crystalline schists 

 would be denser than average molten gabbro under plutonic 

 conditions, although the former are about equal in density to 

 molten basic gabbro. 



It must be considered, too, that even the densest gabbro or 

 peridotite would tend to become somewhat acidified, and thus 

 lose in specific gravity, when in contact with those same silice- 

 ous rocks which alone would float in that magma. Consequently, 

 any immersed blocks would tend to sink to a greater or less 

 depth depending on the extent of the acidification. Secondly, 

 the normal increase of density due to the contact metamorphism 

 of siliceous and calcareous sediments and many crystalline 

 schists invaded by hypogeal magma, must add to the likeli- 

 hood that detached blocks from the country rocks could not 

 remain in suspension in the magma. How significant can be 

 the density increment due to exomorphic changes is illustrated 

 in Rosenbusch's classic researches in the Steiger Schiefer, in 

 Gilbert's account of the contact action about the Henry Moun- 

 tains laccoliths, and in a great number of other studies to which 

 further reference is unnecessary. 



Fluidity of plutonic magmas. — Brdgger has summarized 

 most of the arguments for the general high liquidity of granitic 

 and other deep-seated magmas.* They are so convincing that 

 this question, postponed from an earlier stage in the present 

 line of thought, need not detain us long. The known attributes 

 of those portions of such magmas as reach the surface of the 

 earth as lavas, and the common abundance of thin but oft^n 

 remarkably long, apophysal sheets and dikes from the main 

 intrusive bodies, are facts too well established to permit of 

 essential doubt as to high fluidity. Nothing can seem more 

 probable than that the relatively small fall in temperature, 

 represented in the passage of a thinly molten magma to a 

 toughly viscous condition, has actually taken place in plutonic 

 bodies. Doelter has shown experimentally that that decline 

 in temperature under surface conditions may be from 1240° C. 

 * Die Eruptivgest. des Kristianiagebietes, iii, 336 (1898). 



