STRUCTURE OF LAND AREAS 



79 



ments of other rocks or remains of plants or animals and 

 usually occur in layers or strata. 



Petroleum is probably a result of the accumulation in 

 the sea of layers of animal and plant remains. These 

 were covered by other layers and, during the ages since 

 their formation, they have decomposed and changed into 

 oil and gas. 



The metamorphic rocks have a crystalline structure, 

 often contain well-formed crys- 

 tals imbedded in them and 

 often bands of crystalline sub- 

 stances extending through 

 them. These rocks are not in 

 the condition t in which they 

 were originally laid down, but 

 are modified forms of either 

 the igneous or sedimentary 

 rocks. The rocks originally 

 laid down have been subjected 

 to changes which have rearranged their mineral con- 

 stituents and changed the structure. 



These changes are generally due to heat and pressure. 

 Marble is a crystallized limestone and gneiss generally a 

 metamorphosed granite. Slate and mica-schist are greatly 

 changed clay rocks and anthracite coal is a metamorphosed 

 form of bituminous coal. The rocks of this group are 

 often hard to distinguish from igneous rocks. 



41. Structure of the Land Areas. Experiment 40. Take a 

 copper ball having a ring just large enough to encircle it, the same 

 apparatus as used in Experiment 19. (Fig. 22.) Place the ball 

 within the ring and heat them both to a high temperature. Remove 

 the ball from the ring and plunge it into a dish of water. Place the 

 cooled ball again within the ring. The ring will be found too large 

 to fit snugly upon it. 



If the ring had been a cold hollow sphere fitting tightly to the sur- 



GNEISS. 



Probably metamorphosed 

 granite. 



