STRUCTURE OF LAND AREAS 



255 



GNEISS 

 Probably metamorphosed granite. 



often contain well-formed 

 crystals embedded % in 

 them and often bands of 

 crystalline substances ex- 

 tending through them. 

 These rocks are modified 

 forms of either the igne- 

 ous or sedimentary rocks. 

 The original, igneous or 

 sedimentary rocks have 

 been subjected to forces, 

 such as heat and pressure, that have produced physical and 

 sometimes chemical changes in them. 



Marble is crystallized limestone, and gneiss is generally 

 a metamorphosed granite. Slate and mica-schist are 

 greatly changed clay rocks, and anthracite coal is a metamor- 

 phosed form of bituminous coal. The rocks of this group 

 are often hard to distinguish from igneous rocks. 



Structure of Land Areas. Not only do the land areas 

 differ greatly in the kind of rocks of which they are com- 

 posed, but also in the way in which these rocks are placed. 

 Some of the rocks lie nearly in the condition in which they 

 were originally formed, while others have been folded and 

 warped and twisted. Vast layers of rocks have been worn 

 away by the forces which are continually wearing away and 

 removing the rocks at the surface of the earth, and thus 

 rocks which were once at great depths below the surface 

 have been exposed. Even granite rocks which were origi- 

 nally formed at a depth of thousands of feet below the sur- 

 face now appear at the surface and are being quarried in 

 many places. 



