60 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



Locality. 



British Islands. 

 Appalachian IMts. 

 Deccan, India. 



Great Rift, 



Africa. 

 Washington State, 

 K. W. Scotland. 

 Iceland. 



Washington State. 

 Great Rift, 



Africa. 

 Great Basin, 



U. S. A. 

 Snake River, 



Idaho. 

 Hauran, Syria. 

 Iceland. 



Dale of Fissure Eruption. Preceding Orogenic Period. 



Carboniferous. Devonian. 



Triassic. Close of Paleozoic. 



Cretaceous (or early Late Triassic (also later? ). 



Tertiary ?). 

 Cretaceous (Kaptian " " " " 



series). 

 Eocene (Teanaway basalt). Close of Laramie. 

 Oligocene (Lower Miocene). ? 



Miocene. 1 



Miocene (Yakima basalt). Close of Eocene. 

 Miocene (1) Tertiary (Alps, etc.). 



Pliocene. 



Pliocene. 



Miocene. 



Late Miocene. 



Pliocene. Tertiary. 



Pleistocene and Recenj;. Tertiary. 



Eruption through Local Foundering. 



Narrow abyssal injections, like the average dike located near the 

 earth's surface, will not be expected to show evidences of extensive 

 assimilation of wall-rock, except at levels deeper than those which can 

 be exposed by crustal deformation and erosion. If the width of the 

 injection falls below a certain critical value, the magma is chilled too 

 rapidly to permit of marginal solution or of stoping. If the width ex- 

 ceeds that critical value, the molten or magmatic period is prolonged 

 and absorption of the country-rock by both methods may become im- 

 portant. Other conditions being alike, the critical width where the 

 injection cuts the gneissic pre-Cambrian shell is doubtless somewhat 

 greater than it is where the injection cuts the more hydrous, average 

 sedimentary terrane. But in all cases, the minimum width for abyssal 

 injections which have absorbed large masses of foreign rock, is to 

 be measured in hundreds of meters, if not in kilometers. Abyssal in- 

 jections of greater volume are believed by the writer to work their way 

 upward, for hundreds or thousands of meters, by absorption of the 

 country-rock. The molar-contact absorption may be directly solu- 

 tional, or directly mechanical, i. e., by stoping (followed by solution of 

 the sunken blocks in depth). The evidence for assimilation, either 

 marginal or abyssal, is generally masked by that drastic differentiation 



