GEOLOGY. 259 



prevail in the northern and southern hemispheres, from the elliptical 

 form of the earth's orbit, are periodically transferred by precession 

 from one hemisphere to the other, so that, by an alternate increase 

 and diminution of heat and cold, the frigid zones of each hemisphere 

 are alternately contracted and enlarged in breadth. Thus, during 

 the full course of a precessional cycle, say of twenty-four thousand 

 years, there will occur an extreme variation of tropical latitudes 

 equal to 47, or of from 23^ north to 231 south ; sufficient to have 

 once brought the true line of the tropics within 5 of the southern 

 coast of England. 



Another consequence is, a periodical shifting of the beds of the 

 oceans, from the law which allows fluids more readily than solids to 

 follow the slightest impulse of a new centre of gravity, and from the 

 alternate congealing and liquefying of unequal quantities of water in 

 the Arctic and Antartic Seas ; and a third consequence is, a periodi- 

 cal dislocation of strata from an addition to, or release from, superin- 

 cumbent pressure whenever the waters of the oceans find new beds 

 and channels : a cause alone sufficient to account for the upheaving 

 of mountain chains. 



A general conclusion from the same premises may be drawn, that, 

 from the slow but continuous displacement of the fluids and solids of 

 the globe in progress, the protuberant matter of the equator must be 

 insensibly forming and reforming itself of fresh materials, so that in 

 the lapse of infinite ages every pebble on the earth's surface may 

 take its turn of exposure to tropical and arctic influences, without 

 any further change in the direction of the earth's axis than the pre- 

 cessional change described, and without any alteration in the sphe- 

 roidal form of the earth's mass. 



PHYSICAL GEOGRAPHY OF THE BED SEA. 



The following is condensed from a paper recently read by Dr. 

 Buist to the Bombay Geographical Society, and from remarks ap- 

 pended to it : 



The length of the Red Sea from the Straits to Suez is 1,230 miles ; 

 the greater strait is 13 miles wide, and the lesser li mile. Its entire 

 circuit measured round both gulfs is 4,020 miles; its area, 108,154 

 miles, and its cubical contents probably 800,000 miles. Its greatest 

 breadth, under parallel 17 north, is 192 miles, and it narrows to- 

 ward both extremities. Two-thirds of the Red Sea had never been 

 sounded when this paper was written, and the greatest depth tried 

 was at lat. 25 20', where no bottom was found at four hundred fath- 

 oms. The Gulf of Aden, which continues the communication from 

 the Straits to the Arabian Sea, is a funnel-shaped estuary, above 

 nine hundred miles in length, and nearly two hundred across from the 

 north-west point of Africa to the Arabian shore. 



The name, Red Sea, is derived from large portions being covered 

 with patches, from a few yards to some miles square, composed of 

 microscopic vegetables, or animalcule, particularly abundant in 

 spring, and which dye the water an intense blood-red. When not 

 affected by these organic beings, the deep waters are intensely blue, 

 and the shoal waters shades of green. Contrary to expectation, the 



