GEOLOGICAL HISTORY OF THE EARTH 33 



been found. In certain layers it is found that given 

 kinds of animals or plants are more abundant than in 

 others, and thus definite periods of ages are marked off 

 in successive layers, as the Devonian age, when fish 

 forms of life were most abundant. The Carboniferous 

 age is marked by the enormous growth of lower forms 

 of plants which have given us the large coal fields. But 

 the most important of all the teachings of this wonder- 

 ful history is, that our present forms of plants and 

 animals have ascended from lower, simpler and inferior 

 forms, and that we may expect to find this process still 

 continuing. 



All the things about us that have in no way been 

 modified by man we call natural, and the wonderful 

 things taught by them we call natural history. This 

 natural history is a wonderful book and we see about 

 us in living things and in rocks, hills and rivers 

 its most recently written pages. One of the most in- 

 teresting facts is the evidences in the layers of rock, 

 slate, clay, etc., of the great age of our world, or the long 

 time covered by natural history. There are several 

 theories as to how our planet came to have the shape 

 and size that it has, but there is much evidence that in 

 the beginning the earth was very hot. According to 

 this theory, part of the water now in the seas, rivers and 

 earth was in the atmosphere as gas. As the cycles of 

 centuries rolled on, the earth gradually became cooler. 

 A part of the water fell on the surface. As the surface 

 of the earth lost its heat, it would crack and part of the 

 water would rush in, the heat being so intense as to 

 form such volumes of steam that terrific explosions took 

 place. As the surface cooled sufficiently, it was found 

 that the very first forms of animal and vegetable life 

 appeared, but the earth was not in a settled form even 

 when these appeared, for the geologist finds in the 

 layers of rocks of elevated regions the remains of plants 



