30 ANCIENT PLANTS 



principal ways in which coal was formed, and the dif- 

 ferent seams are illustrations of the products of different 

 methods. In all cases more or less water is required, 

 for coal is what is known as a sedimentary deposit, that 

 is, one which collects under water, like the fine mud and 

 silt and debris in a lake. It will be understood, however, 

 that if the plant remains were collecting at any spot, and 

 the water brought in sand and mud as well, then the 

 deposit could not have resulted in pure coal, but would 

 have been a sandy mixture with many plant remains, 

 and would have resulted in the formation of a rock, such 

 as parts of the millstone grit, where there are many 

 streaks of coal through the stone. 



Among various coal seams, evidence for the following 

 modes of coal formation can be found: — 



[a) In fresh water. — In still freshwater lakes or 

 pools, with overhanging plants growing on the banks, 

 twigs and leaves which fell or were blown into the 

 water became waterlogged and sank to the bottom. 

 With a luxuriant growth of plants rapidly collecting 

 under water, and there preserved from contact with the 

 air and its decaying influence, enough plant remains 

 would collect to form a seam. After that some change 

 in the local conditions took place, and other deposits 

 covered the plants and began the accumulations which 

 finally pressed the vegetable mass into coal. 



To freshwater lakes of large size plants might also 

 have been brought by rivers and streams; they would 

 have become waterlogged in time, after floating farther 

 than the sand and stones with which they came, and 

 would thus settle and form a deposit practically free 

 from anything but plant remains. 



{b) As peat. — Peat commonly forms on our heather 

 moors and bogs to-day to a considerable thickness. This 

 also took place long ago in all probability, and when the 

 level of the land altered it would have been covered by 

 other deposits, pressed, and finally changed into coal. 



{c) In salt or brackish water, growins; in situ. — Trees 



