222 PLANT-LIFE 



issued to its surface by means of volcanoes. Primary 

 rocks yield no fossils. They are crystalline or glass- 

 like; they commonly occur in huge masses, or as sills 

 and dykes, without stratification. It is in the Secon- 

 dary or Stratified rocks that we look for fossils. These 

 rocks are composed of material derived from broken-up 

 primary masses, and they occur in beds or layers ; hence 

 the term " stratified." The gravel, sand, and mud 

 which are being carried by streams into lakes and seas 

 will gradually settle down into beds, and ultimately be 

 consolidated into rocks. The sand will, if not disturbed, 

 become sandstone, and the mud shale. Skeletons of 

 animals and parts of plants included in the deposits may 

 become fossils. Limestones are even now being formed 

 by chemical agency, and also from the mineral remains 

 of corals, molluscs, corallines, etc. What is happening 

 now has happened throughout the ages. Ever since the 

 first land surface was exposed to the elements, the land 

 has been subjected to denudation. Hard rocks have 

 been broken up by frost, eroded by running water, bat- 

 tered by hard fragments borne by wind and wave. 

 Wind has formed dunes from particles of rock, running 

 water has carried pebbles, gravel, sand, and mud, and 

 deposited them in seas and lakes. Earth movements 

 have changed the contour of the land. They have de- 

 pressed land surfaces, and raised ocean bottoms above 

 the level of the water. Beds of stratified deposits 

 formed under water are now exposed and available to 

 human uses. The fossil-hunter finds in them records 

 of the flora and fauna of the ages in which they were 

 deposited. Earth movements are slow and gradual. 

 They are taking place to-day, although the general 



