184 FIRST YEAR COURSE IN GENERAL SCIENCE 



than the rock around it, but the formation is still called a 

 dike. Many chasms seen in the rocks at the seashore are 

 formed by the wearing out of a dike. 



If fissures are filled with minerals left by evaporation from 



solution, the material fill- 

 ing the fissure is called a 

 vein. Many valuable 

 ores are veins. Gold is 

 oftenest found in quartz 

 veins. 



207. The Value of 

 Rocks and Minerals. 

 The crust of the earth 

 is a veritable storehouse 

 of materials from which 

 man has drawn for thou- 

 sands of years. Build- 

 ings still standing in 

 ancient cities, as Athens 

 and Rome, show that 

 more than two thousand 

 years ago men knew how 

 to quarry and cut stone. 

 The pyramids of Egypt 



FIG. 93. A CHASM IN A SEASHORE 

 CLIFF 



The opening between the rocks was 

 once filled with igneous rock. There were 

 many cracks in the dike caused by con- 

 traction in cooling. 1. How did their 

 presence lead to the removal of the dike? 

 2. Will the chasm widen as time goes by? 

 Why? 



and the buried temples 

 and palaces of Babylon 

 are even older. 



The modern cities of 

 Europe and America re- 

 quire immense quantities of stone, which is shipped from re- 

 gions where it is accessible, to the place where it is to be used. 

 The requirements of a good building-stone are firmness of 

 structure, fine grain, and resistance to water and changes of 

 temperature. Sedimentary rocks and rocks which were 

 slowly metamorphosed best fulfill these conditions. 



