192 FIRST YEAR COURSE IN GENERAL SCIENCE 



out of water (perhaps at low tide) and partly dry when the 

 drops fell. The mud must later have been slowly covered 

 and more mud must have been deposited to fill the imprints 

 of the drops. Such relics are usually found in very thin 

 layers of shale. 



By such observations, some of the past history of a con- 

 tinent is learned. It has not always been dry land. Wher- 



FIG. 99. Low TIDE ON A CORAL REEF 



The coral polyps whose skeletons have made these stone-like masses, 

 cannot live many hours out of water. The living polyp is always on the 

 surface of a mass representing many generations of ancestors. 1. Which 

 varieties of coral would resist the action of heavy waves longer, the branch- 

 ing skeletons (stag-horn coral) or massive coral like this? 2. Is this scene, 

 then, probably on the shore of an ocean or an interior sea? 



ever there is sedimentary rock, at some past time there 

 has been water to deposit sediment. Fossils tell us whether 

 it was salt or fresh water. 



212. Changes in the Size of Continents. Study has 

 shown also that the continents have not maintained a 

 steady growth in size by increase around the borders. In 



