REPORT OF THE CHIEF ASTRONOMER 



700 



SESSIONAL PAPER No. 25a 



is available for extrusion. As magma nears the surface, the separation of the 

 dissolved gas must still further increase the volume and tend to cause outflow 

 at the surface. The relative importance of these three conditions for extrusion 

 is by no means apparent. 



The fact that the great bulk of visible igneous rock is intrusive, and the 

 related fact that most of the larger Paleozoic and later injections have not 

 extended to the surface, suggest that the upper part of the Earth's crust has long 

 been comparatively difficult of penetration by abyssal magma. It seems fair to 

 hold that a leading cause of this relative impenetrability is the state of compression 

 in the outermost shell of the crust. The compressive stress is relieved by an 

 orogenic paroxysm. After each paroxysm tensions in the same shell are pro- 

 duced by the cooling of the rocks which have been heated by shearing. For a 

 double reason, therefore, fissure eruptions should be more numerous and of 

 greater volume in periods subsequent to strong mountain-building. This expec- 

 tation is fairly matched by the facts of geological history, as shown in the- 

 accompanying table: — 



Locality. 



Date of Fissure Eruption. 



Preceding Orogenic Period. 





Middle Cambrian ? 





Rocky Mts. at 49th Parallel 



Early Middle Cambrian £- 



British Islands 



Cretaceous (or Early Tertiary ?) 



Appalachian Mts 



Close of Paleozoic. 



Great Rift, Africa 



Late Triassic (also later ?) 



N. W. Scotland 



Iceland 



Oligocene (Lower Miocene) 



Close of Laramie. 



9 



9 



Washington State 







Great Rift, Africa 





Tertiary (Alps, &c.) 



Great Basin, U.S. A 



Snake River, Idaho 







Hauran, Syria 





Tertiary. 



Iceland 











Eruption through Local Foundering. — When the width of an abyssal 

 injection is increased beyond a critical value, the body may have sufficient 

 thermal energy to enlarge its chamber by incorporating roof and wall rocks. 

 The result is a batholith. The average batholith, at expose'd levels, is granitic 

 because granite represents the stable and least dense differentiate of the average 

 8yntectic. 



The integrity of the batholith's roof is evidently threatened ' in two ways. 

 It is thinned during the absorption of the roof-rock by the molten magma. The 

 latter might work its way to the surface through piecemeal stoping, which 

 should continue until a large area of the batholithic roof has disappeared. On 

 the other hand, it is also possible that part or all of the roof might, under special 

 conditions, founder en masse in the less dense magma. In either case true 

 volcanic action is produced. Such wholesale or piecemeal foundering woukL 



25a— vol. iii— 46J 



