Page:Popular Science Monthly Volume 70.djvu/405

Rh the River Cobre to the carriage road. From soundings taken by the kindness of Mr. Charlton Thompson, harbor master, it was ascertained that in several places along the edge of the harbor, the bottom had sunk from old soundings of a fathom and a half to over six fathoms, and that on the harbor side of the base of the Palisadoes a series of step-faults reached a maximum depression at the shore to the north of four fathoms (Figs. 12 and 13). This zone of disturbance continued, as far as could be traced, in an interrupted line along the Palisadoes, and caused a maximum depression at the western tip of Port Royal, where the buildings were tilted by the sinking and a hundred yards or more of land were submerged to a depth of from eight to twenty-five feet (Figs. 14-16). This Assuring of the earth was caused by the repeated tearing apart and closing of the earth's crust, accompanied generally by the ejection of water, sand and mud, sometimes to the height of three or four feet, but the subsidence prevented the forming of any cones about these craterlets. The sands first thrown up were afterwards covered by a layer of mud.

To account for the unique line of Assuring and subsidence is difficult. It was noted that considerable disturbance took place at the shore line where the earth vibrations were refracted in changing from the medium of one elasticity to a medium of a different elasticity. But the middle portions of the harbor were stable and the channel was unchanged, though a beacon light near Fort Augusta was broken off. In this limestone country, solution by underground waters might be sufficient to account for the sinking of a small area like the harbor ac Kingston. But the harbor did not sink—only a small encircling zone, and that located either on the shore or slightly offshore. The continuous tearing apart and closing of these fissures, covering a few hours' time as it did in some instances, might account for the hydraulicking of the loosely compacted sands and gravels in the zone of Assuring, and allow subsidence. Again, ground-waters may have caused considerable solution of the limey constituents where the waters entered the harbor. No theory as yet satisfactorily accounts for this peculiar subsidence. At the eastern end of the harbor at Rock Fort a considerable change in underground drainage was observed, where a small spring was increased to a stream eight feet wide and six inches deep.

It was here at the Rock Fort penitentiary quarry that a guard gave me the only reliable account of a sea wave. After a few moments had elapsed and the convicts had run from the landslides on the face of the quarry and gathered around him for protection, the sea retreated for a hundred feet and then advanced inward upon the shore about sixty feet in a low wave a couple of feet high. Ocho Rios, near