1889.] XEW YORK ACADEMY OF SCIENCES. 17 



back than this. Given a system of protaxial lines, and the 

 growth of the continent will follow and conform to them. Of 

 course, countless modifications will arise, and interfere with any 

 mathematical regularity of form ; apart from tiiese, however, 

 the great facts are as stated. But what laws or forces have, in 

 the early stages of crust-growth, given direction to these diagonal 

 protaxes ? This is the problem for which I venture to suggest a 

 solution. 



Much has been said and written about strains in the crust 

 arising from various sources. Mr. T. Mellard Eeade, e. g., has 

 laid great stress upon difference of temperature between the 

 outer and the inner portions of the crust, regarding the latter as 

 tending to expand by heat, while held in compression from with- 

 out by the cooler and contracting envelope of the super-crust. 

 The more general view has regarded the entire crust proper as 

 tending to contract irregularly upon the shrinking interior mass. 

 But whatever our view as to the process of deformation by the 

 elevation and depression of portions of the surface, we are 

 brought back to the same question as to the origin of these 

 diagonal lines, to which the irregularities of criist-level have so 

 largely tended to conform. 



I desire to call attention, at least as a subject worthy of in- 

 quiry, to the horizontal strains resulting from the differential 

 velocities in the rotation of the earth's crust. From an east- 

 ward velocity of zero at the poles, to that of a thousand 

 miles an hour at the equator, the actual speed of rotation in- 

 creases at a rate which, were the earth a perfect sphere, would 

 vary as the cosine of the angle of latitude. The centrifugal or 

 inertial force due to this increasing velocity, will vary as the 

 square of the actual rate of motion, and has by some been re- 

 garded as competent of itself to produce the present amount of 

 polar flattening, even in the earth's existing condition, and 

 apart from a former supposed fluidity of the mass. The full 

 discussion of these topics, however, belongs to mathematical 

 physics, and cannot be entered upon here, at least for any quan- 

 titative estimates. All that is attempted in this paper is to point 

 out the tendency of these differential velocities to ])roduce in the 

 crust a series of strains, which would of necessity take diagonal 

 courses such as those referred to. 



In illustration of this view, I recall for comparison the pheno- 

 mena observed in the ice of moving glaciers, as expounded by 

 Tyndall and other leading glaciologists. The formation of two 

 intersecting systems, diagonal to the glacier and at right angles 

 to each other, — one of ridges and one of crevasses, — is of course 

 familiar to all present, as resulting from the difference of velocity 

 between the central and the marginal regions. A " shearing" 



