Page:Vol 6 History of Mexico by H H Bancroft.djvu/695

Rh 5. The 24-foot curve of depth has neither shifted nor sensibly altered its shape. It is inside of the bar, convex toward the sea, and its apex is nearly tangent to the coast line.

6. Between the apex of this curve and the southern edge of the bar, great changes have taken place. The channel has become narrower, and though much deeper, it is quite crooked in the line of maximum depth.

7. No results have reached my hands of the nature of the drift upon the bar before 1840. Now it is made up of coarse and fine, gray and black, sands, shells, and clay, with red and black specks.

8. The black sands belong to the Uspanapa, and, as has been remarked, can be prevented from reaching the harbor by inducing the formation of a bar at the mouth of the Uspanapa, in front of which the sands will naturally settle, practically for an indefinite period.

9. The Coatzacoalcos Bar is an external bar, and is often disturbed by northers. Since the drift, held in suspension by the agitated sea, cannot run up against the Coatzacoalcos current, it is deposited outside of and parallel to the coast, especially toward the eastern side of the entrance. This conclusion seems plausible from the shape of the 6-foot curve.

10. The 18-foot curve shows the limiting height of greatest deposits, and it has advanced toward the channel with an irregular serrated outline, confining the effective cross-section of the river; consequently, the increased depth of water over the bar is accounted for, because the increased velocity, due to the stricture in the channel, has blown away the fine sands.

11. The bar is permanent, with a clay subsoil. This can be shown by the 18-foot curve, thus: The nature of the deposits should be considered with regard to their cohesion and density; as is shown by the slopes of the profiles, the particles deposited over the lower portion of the river slide easily upon each other, and are of light weight. If the bar and river deposits were of uniform density, the current would cut a channel with sensibly parallel outlines; but if the bar is made up of a substance harder than the drift upon it, it will act as a dam, against which the current will impinge, and becoming divided and thrown sideways, it will excavate irregular channels through the material that collects in front of the bar. Also, since the edge of the bar acts as the lips of a dam, the current will wash away the light particles, no drift will be found upon it, and eddies will be found in close proximity to the obstructions, which will produce both shallow and deep holes, not far removed from each other.

If we look at the chart of 1871, it will be seen that these effects, which are the irrevocable sequence of dynamical laws, obtain in the Coatzacoalcos Bar in a very remarkable manner; hence they must be attributed to the hardness and permanence of the bar. It will also be observed that those points of the bar which should expose the subsoil to the action of the currents are the only places where the sound gives 'hard clay.'

12. The profiles and sections accompanying the chart of 1871 will give an accurate idea of the present state of the bar. The parts ruled in black lines represent the required amount of dredging, under the supposition that the entrance channel will be 1,200 feet wide at the bottom throughout its length, and 25 feet deep from the entrance until after crossing the present bar.