344 fossils 



Again, the conditions under which organisms live have a great 

 influence upon the chances of their preservation as fossils. Land 

 animals and plants are much less favourably situated than are 

 aquatic forms, and since by far the greater number of sedimentary 

 rocks were laid down in the sea, marine organisms are much more 

 common as fossils than are those of fresh water. 



Few rocks, and those unimportant, are accumulated on land, 

 and so fossils are rarely preserved on land surfaces, though they are 

 occasionally buried in blown sand or in the soil. Peat bogs are, 

 however, excellent places for fossilization, and the coal seams 

 have yielded great numbers of fossils, principally of plants. The 

 remains of land animals and plants, especially of the latter, are 

 sometimes swept out to sea, sink to the bottom, and are there 

 covered up and preserved in the deposits ; but such occurrences 

 are relatively uncommon. Lakes offer much more favourable 

 conditions for the preservation of terrestrial organisms. Sur- 

 rounding trees drop their leaves, flowers, and fruit upon the flats 

 and shallows of fine mud, insects fall into the quiet waters, 

 while quadrupeds are mired in mud or quicksand and soon 

 buried out of sight. Flooded streams bring in quantities of 

 vegetable debris, together -with the carcases of land animals, 

 drowned by the sudden rise of the flood. When the carcase of 

 a freshly drowned mammal is washed into the lake, it immedi- 

 diately sinks to the bottom ; and if sufficient sediment be de- 

 posited upon it, it will be held there and fossilized as a complete 

 skeleton. But if little sediment be thrown down upon it, the 

 carcase will rise and float, when the body is distended by the gases 

 of decomposition. Floating thus, it will be pulled about by the 

 flesh-eating creatures which inhabit the lake, dropping one bone 

 here and another there, over a wide area. 



The great series of lake deposits, which for long ages were 

 formed in various parts of our West, have proved to be a mar- 

 vellous museum of the land and fresh-water life of that region. 

 On the fine-grained shales are preserved innumerable insects and 

 fishes, with multitudes of leaves, many fruits and occasionally 

 flowers, while in the sands and clays are entombed the bones of 



