8 GEOLOGY OF THE ISLE OF WIGHT 



deposit of carbonate of lime from sea-water containing 

 more of the soluble bicarbonate than it can hold. 



These limestones are examples of rocks which are said 

 to be of organic origin, that is to say, they are formed by 

 living things. Organic rocks may be formed by animal 

 or vegetable growth. Rocks of vegetable origin are seen 

 in the coals. A peat bog is composed of a mass of vege- 

 table matter, chiefly bog moss, which for centuries has 

 been growing and accumulating on the spot. At the 

 bottom of the bog will frequently be found trunks of oak, 

 or other trees, the remains of a forest of former days. The 

 wood has undergone chemical changes, has lost much of 

 its moisture, and often become very hard, as in bog oak. 

 Beds of coal have been formed by a similar process, on a 

 much vaster scale, and continued much longer. The 

 remains of ancient forests have been buried under sand 

 stones and other rocks, have undergone chemical change, 

 and been compressed into the hard solid mass we call 

 coal. Fossil wood, which has not reached the stage of 

 hard coal, but forms a soft brown substance, is called 

 lignite. This is of frequent occurrence in various strata 

 in the Isle of Wight. 



Of organic rocks of animal origin the most remarkable 

 are the chalk, of which we shall speak later, and the coral- 

 reefs, which are found in the warm waters of tropical seas. 

 Sailing over the South Pacific you will see a line of trees 

 coco-nut trees chiefly looking as if they rose up from the 

 sea. Coming nearer you see that they grow on a low 

 island, which rises only a few feet above the water. These 

 islands are often in the form of a ring, and look "like 

 garlands thrown upon the waters." Inside the ring is a 

 lagoon of calm water. Outside the heavy swell of the 

 Southern Ocean thunders on the coral shore. If a sound- 

 ing line be let down from the outer edge of the reef, it 

 will be found that the wall of coral goes down hundreds 

 of feet like a precipice. On an island in the Southern Sea, 



