356 PRACTICAL LESSONS IN SCIENCE. 



fine clay thick enough to hold water and support a growth of 

 vegetation ; a slight elevation makes shallow, stagnant or slow- 

 moving water, in which springs up a dense growth of mosses, 

 ferns, equisetse, lycopodia, sigillaria, calamites, etc., forming 

 a swamp. The stems and leaves of these plants, preserved from 

 complete decay by the water, accumulate as peat. After a time 

 the area is slightly depressed and currents of water flow over the 

 peat, depositing on it sediments of clay or sand, and perhaps the 

 depression may be enough to allow the sea to invade the region 

 and deposit lime sediments. Again the region is elevated, a de- 

 posit of fine clay is made and the cycle of deposits begins anew. 

 In Belgium, Nova Scotia and other localities there are more than 

 eighty seams of coal, and as at least two strata of other material 

 separate each seam from the next, we can form some idea of how 

 many upward and downward movements, how many changes of 

 condition have occurred in the for mat ion of a group of coal strata. 

 The layers of peat are changed by heat and pressure into bitu- 

 minous coal, and the pressure and heat maybe enough to change 

 the beds of peat to anthracite coal, or even to graphite. 



Iron in some form always occurs with coal. Nearly all the iron 

 of England, and much of the iron of America, comes from the 

 coal measures. It often occurs as the sulphide, which is of no 

 value for iron, and is a damage to the coal ; but it occurs most 

 abundantly as clay iron stone, or kidney iron ore, which is a car- 

 bonate of iron. In some manner, not well understood, vegetation 

 gathers up the iron from adjacent rocks, and concentrates it into 

 these forms. Sand rocks and shales near coal seams are white or 

 gray, not red their coloring matter has become iron ore. 



Besides coal, we find in the rocks of different ages, quantities 

 of asphalt and bitumen, petroleum oil and natural gas. They 

 are supposed to be of organic origin, as similar substances may 

 be derived from coal by distillation ; and while they might origi- 

 nate from any accumulation of organic matter, it is probable that, 

 for the most part, they were the products of the carboniferous 

 age, although they may have accumulated in cavities of rocks of 

 other ages. 



