Page:Popular Science Monthly Volume 53.djvu/20

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10
THE POPULAR SCIENCE MONTHLY.
THE WEST INDIAN BRIDGE BETWEEN NORTH AND SOUTH AMERICA.
By Dr. J. W. SPENCER.

MUTABILITY OF OCEANS AND CONTINENTS.—A glance at a map of the American continent, inclosing the West Indian seas within its mass, suggests that these basins are sunken plains, submerged to only a moderate extent, but the soundings show depths reaching to more than three miles. "It is not too much to say that every spot which is now dry land has been sea at some former period, and every part of space now covered by the deepest oceans has been land."[1] This enunciation still held place among the latest writings of the great geological teacher—Sir Charles Lyell. As the earlier geologists had not the means of measuring the amount of terrestrial movements, the doctrine of mutability of continents and seas, as taught by Lyell, was doubted by many who later substituted the hypothesis of their permanency from the most remote times, although subjected to ceaseless changes of form. The hypothesis of permanency of continents and seas was largely based upon the littoral character of sedimentary formations, although the evidence of the abysmal or oceanic origin of the widespread chalk deposits could not be easily disposed of. Again, the development and distribution of animal and plant life have been skillfully used as evidence against certain great changes in insular and continental connections, beyond limited proportions. The amount of the concession has varied greatly among the different advocates, so that even under the general hypothesis of permanency, the configuration of the West Indian region has undergone great changes, yet not sufficient to bridge over the seas between the two Americas.

The biological evidence alone, in favor of the permanency of continents and oceans, sometimes suggesting the most startling evidence to the contrary, is insufficient to base theories upon, unless supported by physical indorsement. Indeed, some of the most interesting questions concerning the distribution of biological forms in the West Indian seas can only be explained by the recent discoveries of the physical changes of the region. Some of the smaller oscillations of land and sea have often been measured, but the determination of many of those of stupendous proportions, while occasionally hinted at, was not accomplished until the standard value of the great yardstick was found in the West Indian seas. The discovery of the Antillean bridge between North and South America, which is the

  1. The last edition of Lyell's Principles of Geology.