296 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 19 2 8 



As the sinking: strain slowly increases in the outer shell, the imme- 

 diate cause of its release — the trigger that sets the mechanism in 

 motion — may be a compression of the earth's crust such as is mani- 

 fested elsewhere in a mountain-making movement. To illustrate 

 this effect of compression, take two laths and support them in hori- 

 zontal position at the ends. Load one of the laths in the middle 

 with a few pounds of weight. The load will produce only a slight 

 elastic flexure. Now compress both with a horizontal thrust. The 

 loaded lath will bend doAvn and break under a slight compression 

 which will hardly affect the other. Thus a movement of crustal 

 compression acting horizontally through the whole lithosphere will 

 tend to vertical warping of all tracts not in isostatic equilibrium, 

 either upAvard or downward, according to the direction of the pre- 

 existing vertical stress. The surface expression of such warping is 

 likely to be manifested as great fractures between blocks of crust. 

 These faults pass miles below into broad flexures. 



During the Middle and Upper Miocene occurred the final foun- 

 dering of Eria, and also one of the greatest mountain-making 

 movements since the pre-Cambrian, A sufficient geological interval 

 had elapsed after the early Oligocene igneous activity of Eria for 

 the solidification and attendant increase in density of the basic 

 magmas, with consequent development of downward stresses. It is 

 believed that in this way Eria was broken up and large parts of 

 it foundered into the Atlantic Ocean. 



The hypothesis of the relationship of continental fragmentation 

 to basic intrusions on a large scale, reconciles the apparent con- 

 flict with isostasy and brings together harmoniously in the explana- 

 tion of the foundering of Eria a number of otherwise apparently 

 unrelated phenomena. Measured by this test the hypothesis is suc- 

 cessful. 



The example is, however, geologically recent and much of the 

 evidence is preserved. The land bridge has broken down, but its 

 pierlike remnants still stand as horsts in the ocean. In certain 

 other cases the evidence is more or less indeterminate, as indeed is 

 to be expected. 



In northwestern India, the Deccan traps outcrop on the coast 

 from latitude 17° N. to the Tropic of Cancer (about 22°), and 

 extend inland halfway across the peninsula, occupying to-day 

 200,000 square miles and forming one of the greatest basalt fields 

 of the continents. [According to Wadia,^° however, the outliers 

 in greater India indicate that the original flows covered an area of 

 500,000 square miles.] They are broken off by the shore line, the 

 nearly horizontal beds rising in terraces or steps to elevations of 



IS D. N. Wadla, Geology in India, pp. 192-201. 1919. 



