158 THE DEPTH AND MARINE DEPOSITS OF THE PACIFIC OCEAN. 



approximate areas covered by the different shades of color, and the per- 

 centages to the total area : — 



Area covered by palest shade denoting less than 25% CaCOg - 

 Area covered by second shade denoting 25% to 50% CaCOs . 

 Area covered by third shade denoting 50% to 75% CaCOg . . 

 Area covered by darkest shade denoting more than 75% CaCOa 



This table indicates that by far the largest area is covered by the shade 

 denoting less than 25 per cent of calcium carbonate, which is about six times 

 greater than the area covered by the next shade (25 to 50 per cent), five 

 times greater than the area covered by the third shade denoting 50 to 

 75 per cent, and ten times greater than the area covered by the darkest 

 shade, indicating over 75 per cent of calcium carbonate. This lightest shade 

 of green covers nearly the whole of the North Pacific as well as a large part 

 of the South Pacific. In the North Pacific there are only three large areas 

 covered by the darker shades of green, viz., one surrounding the Caroline 

 Islands, a second surrounding the Marshall and Gilbert archipelagos, and a 

 third stretching from 137° to 163° west longitude and from 9° north to 3° 

 south latitude, with many small areas surrounding coral islands and on sub- 

 marine elevations. In the South Pacific, on the other hand, there is a large 

 dark-colored area consisting of two portions, one in the east and one in the 

 west, joined by a narrow band along the south, and stretching from 58° 

 south to 7° north latitude. These portions are separated by a central area 

 of lightest green, and while the western one abuts on the Great Barrier Reef 

 of Australia, the eastern one is separated from the coast of South America 

 by an area covered by the lightest color. 



The second shade of green, denoting 25 to 50 per cent of calcium carbo- 

 nate, covers a smaller area than the third shade, indicating 50 to 75 per 

 cent of calcium carbonate, forming generally an irregular border to the dark 

 areas where the percentage exceeds 50. 



On comparing this map with the depth map it will be observed that the 

 higher percentages of carbonate of calcium correspond generally with the 

 moderate depths of the ocean and to the shore regions where coral reefs pre- 

 vail, while the lower percentages correspond with the very deep water 

 and to the shallow water around the continents and islands not of coral 

 formation. 



