SUBOCEANIC STRUCTURE 299 



would thus have the form of an inverted truncated cone, the apex being 

 turned toward the center of the earth. 



The development of horizontal foliation due to overloading at the sur- 

 face and the formation of marginal shearing surfaces in process of sub- 

 sidence would direct later extrusions toward the margins and would 

 widen the cones, particularly in their upturned bases. They would there- 

 fore depart widely from a linear, conical shape. The elements would be 

 in general irregular curves, formed of zigzag details, as the magma 

 passed from dike to sheet and from sheet to dike, spreading outward and 

 upward. It is evident that the process would tend to broaden an ocean 

 basin at the expense of the continental platform, by invading the margins 

 of the latter with heavy rocks. I regard the Columbia lava flows and 

 others of a similar extensive character as invasions of this nature. 



There are probably limitations of oceanic spreading. There are cer- 

 tainly offsetting processes due to compression; but it is not proposed to 

 treat them here. 



The object of this immediate section is to show reason why the deeper 

 oceanic underbodies should be regarded as the sources of those eruptives 

 which appear at the surface so commonly around their margins. The 

 reason is found, according to the argument, in the origin of the basin, 

 which is regarded as having developed in consequence of the rise of basic 

 magmas and the subsidence of their cooled masses as the overloaded 

 gneissic shell flowed out from under them, chiefly by recrystallization.^^ 



In conclusion, the reader is invited to read over the advanced sum- 

 mary in which the several concepts presented in the article are stated 

 without argument and stripped of details. 



" Note. — It Is but just to my friend, Joseph Barrell, and to myself to call attention 

 to the parallelism which exists between the hypothesis of the origin of ocean basins here 

 set forth with that which is stated in his posthumous paper, "Evolution of the earth" 

 (November, 1916, page 42). We agree as to the I'ise of basic magmas and the resulting 

 conditions of isostatic adjustment. We differ as to the cause of the melting of the 

 magma, which he ascribes to the heat generated by radioactivity. Our paths of thought 

 converge to a common point, yet our thinking had been independent. We had not con- 

 ferred on the subject and my manuscript was in essentially its present form when I 

 read his printed page, in September, 1919. 



