297 [Shaler. 



demonstrated to be entirely Inefficient to produce any thing but the 

 most inconsiderable movement, and that the effects are of a totally 

 different nature from any which could be produced by such means.* 



Although it is not yet possible for us to unravel the complicated 

 problems involved in the changes of oceanic currents in former geolog- 

 ical periods, we may still be able to form some general conception of 

 the character of these streams in certain conditions of the surface, 

 ■which will aid to a slight extent our understanding of past changes of 

 the earth's surface. 



It is eminently probable that the first condition of the ocean was 

 that of a nearly, if not quite unbroken expanse of a much more uni- 

 form depth than is presented by the sea areas of the present day. 

 This being the case, we would have a far less complicated system of 

 oceanic circulation than at the present time. The absence of land 

 areas would be attended by a great equality in atmospheric move- 

 ments. The trade winds, which in the present condition of the earth's 

 surface are greatly disturbed by the. action of the land, would in 

 an unbroken ocean have every where the same regular character 

 which they now exhibit only in the Pacific Ocean, at considera- 

 ble distances from the shore. This would give to the great equatorial 

 movement, the source of all oceanic streams, the character of a great 

 encircling current moving for its whole course within, or nearly 

 within, the tropical limits. The regions to the north and south of the 

 belt of trade winds being, as now, the seat of conflicting atmospheric 

 movements, we could have no definite oceanic currents other than 

 the single intertropical stream. The movement of water from the 

 equator to the poles, to compensate for the surface flow of water 

 towards the equator, due to the meridional element of the motion of 

 the trade winds, would be accomplished most probably by a general 

 movement of the deeper waters rather than by defined currents. In 

 this case the rate of motion of this counter current would be so slight 

 that it could have no considerable influence on the distribution of 

 life or sedimentary materials, and comparatively little effect on the. 

 equalization of the distribution of heat. 



If we consider the effect of ocean currents when complicated by 

 the action of land masses, as at the present day, their varied charac- 

 ter, important influence on the distribution of heat, and effect on rain- 

 fall, we perceive that the transition from the condition of a single 

 equatorial current to the existing complicated system of streams could 

 not be without an important effect upon all those circumstances Avhich 

 regulate the distribution of life, or the deposition of sedimentary 

 materials. AVhlle the tropical current continued unbroken, the 

 oceanic movements could exercise but little influence on the distribu- 



*See J. F. W. Herschel, Phys. Geog. Edinburgh. 1862. p. 52. 



