THE COMPOSITION OF THE EARTH 



45 



B. META-IGNEOUS SERIES 



Only the leading varieties of metamorphic rocks have been 

 described. There are many other kinds which cannot be 

 considered here. The general relations of those discussed to 

 the rocks from which they are commonly derived are shown 

 in the preceding table. 



The relation of rocks to one another. At the very outset 

 the student is likely to encounter rocks that cannot be iden- 

 tified readily with any of the kinds enumerated in the pre- 

 ceding pages. Thus, a rock may be found which contains 

 both sand and calcite in perhaps nearly equal proportions, and 

 which therefore combines the features of a sandstone and a 

 limestone. Varieties of rocks are, in fact, not definite species, 

 as are most kinds of animals and plants. Rather, they grade 

 into each other by imperceptible stages. By a gradual de- 

 crease in quartz, granite verges toward syenite. By an in- 

 crease in hornblende and a decrease in feldspar, syenite passes 

 into diorite. By a decrease in the size of its pebbles, con- 

 glomerate approaches sandstone. Similar transitions occur 

 between all related varieties of rocks. 



Furthermore, rocks change after they have been made, 

 and this produces further gradations from one kind to another. 

 Thus, as noted above, granite may be slowly altered into 

 gneiss, and shale into slate, and slate, in turn, to schist. Shale 

 and schist are distinct in appearance and constitution, yet 

 all possible gradations may be found between them. It is 

 evident, then, that rock names must be used loosely, and that 

 there are few sharp dividing lines anywhere in the classifica- 

 tion. 



